Manufacturing method of semiconductor device

ABSTRACT

In a semiconductor device including an oxide semiconductor, a change in electrical characteristics is inhibited and reliability is improved. The semiconductor device is manufactured by a method including first to fourth steps. The first step includes a step of forming an oxide semiconductor film, the second step includes a step of forming an oxide insulating film over the oxide semiconductor film, the third step includes a step of forming a protective film over the oxide insulating film, and the fourth step includes a step of adding oxygen to the oxide insulating film through the protective film. In the first step, the oxide semiconductor film is formed under a condition in which an oxygen vacancy is formed. The oxygen from the oxide insulating film fills the oxygen vacancy after the fourth step.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/849,852, filed Sep. 10, 2015, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2014-186092 on Sep. 12, 2014, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a manufacturingmethod of a semiconductor device including an oxide semiconductor film.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. In particular, one embodiment of the presentinvention relates to a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a power storage device,a memory device, a driving method thereof, or a manufacturing methodthereof.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as a field-effect transistor (FET) or a thinfilm transistor (TFT)). Such transistors are applied to a wide range ofelectronic devices such as an integrated circuit (IC) and an imagedisplay device (display device). A semiconductor material typified bysilicon is widely known as a material for a semiconductor thin film thatcan be used in a transistor. As another material, an oxide semiconductorhas been attracting attention (e.g., Patent Document 1).

Furthermore, for example, Patent document 2 discloses a semiconductordevice in which, to reduce oxygen vacancies in an oxide semiconductorlayer, an insulating layer which releases oxygen by heating is used as abase insulating layer of the oxide semiconductor layer where a channelis formed.

In addition, a method for manufacturing a semiconductor device in whichan oxide semiconductor layer is highly purified in the following manneris disclosed: an oxide insulating layer is formed over the oxidesemiconductor layer; oxygen is introduced (added) through the oxideinsulating layer; heat treatment is performed; and impurities such ashydrogen, moisture, a hydroxyl group, or hydride are removed from theoxide semiconductor layer by the introduction of oxygen and the heattreatment (e.g., Patent Document 3).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2006-165529

[Patent Document 2] Japanese Published Patent Application No.2012-009836

[Patent Document 3] Japanese Published Patent Application No.2011-199272

SUMMARY OF THE INVENTION

In the case where a transistor is manufactured using an oxidesemiconductor film for a channel region, oxygen vacancies formed in thechannel region of the oxide semiconductor film adversely affect thetransistor characteristics; therefore, the oxygen vacancies cause aproblem. For example, oxygen vacancies formed in the channel region ofthe oxide semiconductor film are bonded to hydrogen to serve as acarrier supply source. The carrier supply source generated in thechannel region of the oxide semiconductor film causes a change in theelectrical characteristics, typically, a shift in the threshold voltage,of the transistor including the oxide semiconductor film. Further, thereis a problem in that electrical characteristics fluctuate among thetransistors. Therefore, it is preferable that the amount of oxygenvacancies in the channel region of the oxide semiconductor film be assmall as possible.

In view of the above problem, an object of one embodiment of the presentinvention is to inhibit a change in electrical characteristics and toimprove reliability in a semiconductor device including an oxidesemiconductor. Another object of one embodiment of the present inventionis to provide a manufacturing method of a semiconductor device includingan oxide semiconductor in which a change in electrical characteristicsis inhibited and reliability is improved. Another object of oneembodiment of the present invention is to provide a manufacturing methodof a semiconductor device with low power consumption. Another object ofone embodiment of the present invention is to provide a novelsemiconductor device. Another object of one embodiment of the presentinvention is to provide a method for manufacturing a novel semiconductordevice. Another object of one embodiment of the present invention is toprovide a novel display device.

Note that the description of the above objects does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all of these objects. Other objects areapparent from and can be derived from the description of thespecification and the like.

One embodiment of the present invention is a manufacturing method of asemiconductor device including a first step, a second step, a thirdstep, and a fourth step. In the manufacturing method, the first stepincludes a step of forming an oxide semiconductor film, the second stepincludes a step of forming an oxide insulating film over the oxidesemiconductor film, the third step includes a step of forming aprotective film over the oxide insulating film, and the fourth stepincludes a step of adding oxygen to the oxide insulating film throughthe protective film. In the first step, the oxide semiconductor film isformed under a condition in which an oxygen vacancy is formed. Theoxygen from the oxide insulating film fills the oxygen vacancy after thefourth step.

One embodiment of the present invention is a manufacturing method of asemiconductor device including a first step, a second step, a thirdstep, a fourth step, and a fifth step. In the manufacturing method, thefirst step includes a step of forming an oxide semiconductor film, thesecond step includes a step of forming an oxide insulating film over theoxide semiconductor film, the fifth step includes a step of heating theoxide insulating film, the third step includes a step of forming aprotective film over the oxide insulating film, and the fourth stepincludes a step of adding oxygen to the oxide insulating film throughthe protective film. In the first step, the oxide semiconductor film isformed under a condition in which an oxygen vacancy is formed. Theoxygen from the oxide insulating film fills the oxygen vacancy after thefourth step.

One embodiment of the present invention is a manufacturing method of asemiconductor device including a first step, a second step, a thirdstep, a fourth step, a fifth step, and a sixth step. In themanufacturing method, the first step includes a step of forming an oxidesemiconductor film, the second step includes a step of forming an oxideinsulating film over the oxide semiconductor film, the fifth stepincludes a step of heating the oxide insulating film, the third stepincludes a step of forming a protective film over the oxide insulatingfilm, the fourth step includes a step of adding oxygen to the oxideinsulating film through the protective film, and the sixth step includesa step of removing the protective film. In the first step, the oxidesemiconductor film is formed under a condition in which an oxygenvacancy is formed. The oxygen from the oxide insulating film fills theoxygen vacancy after the fourth step.

One embodiment of the present invention is a manufacturing method of asemiconductor device including a first step, a second step, a thirdstep, a fourth step, a fifth step, a sixth step, and a seventh step. Inthe manufacturing method, the first step includes a step of forming anoxide semiconductor film, the second step includes a step of forming anoxide insulating film over the oxide semiconductor film, the fifth stepincludes a step of heating the oxide insulating film, the third stepincludes a step of forming a protective film over the oxide insulatingfilm, the fourth step includes a step of adding oxygen to the oxideinsulating film through the protective film, the sixth step includes astep of removing the protective film, and the seventh step includes astep of forming a nitride insulating film over the oxide insulatingfilm. In the first step, the oxide semiconductor film is formed under acondition in which an oxygen vacancy is formed. The oxygen from theoxide insulating film fills the oxygen vacancy after the fourth step.

One embodiment of the present invention is a manufacturing method of asemiconductor device including a first step, a second step, a thirdstep, a fourth step, a fifth step, a sixth step, a seventh step, aneighth step, a ninth step, and a tenth step. In the manufacturingmethod, the eighth step includes a step of forming a gate electrode, theninth step includes a step of forming a gate insulating film over thegate electrode, the first step includes a step of forming an oxidesemiconductor film over the gate insulating film, the tenth stepincludes a step of forming a source electrode and a drain electrode overthe oxide semiconductor film, the second step includes a step of formingan oxide insulating film over the oxide semiconductor film, the sourceelectrode, and the drain electrode, the fifth step includes a step ofheating the oxide insulating film, the third step includes a step offorming a protective film over the oxide insulating film, the fourthstep includes a step of adding oxygen to the oxide insulating filmthrough the protective film, the sixth step includes a step of removingthe protective film, and the seventh step includes a step of forming anitride insulating film over the oxide insulating film. In the firststep, the oxide semiconductor film is formed under a condition in whichan oxygen vacancy is formed. The oxygen from the oxide insulating filmfills the oxygen vacancy after the fourth step.

In each of the above-described structures, the oxide semiconductor filmis preferably formed with a sputtering apparatus in the first step, andthe condition in which the oxygen vacancy is formed is preferably anoxygen partial pressure in the sputtering apparatus of higher than 0 %and lower than 50 %.

In each of the above-described structures, a step of heating the oxidesemiconductor film is preferably further performed after the first step,and an oxygen vacancy is preferably formed in the oxide semiconductorfilm in the step of heating the oxide semiconductor film. Moreover, thestep of heating the oxide semiconductor film is preferably performed inan atmosphere in which an oxygen partial pressure is lower than or equalto 1%.

In each of the above-described structures, the step of adding oxygen ispreferably performed with a plasma treatment apparatus.

In each of the above-described structures, the protective filmpreferably includes at least one element selected from In, Zn, Ga, Sn,Ti, Al, W, Ta, Mo, and Si.

In each of the above-described structures, the oxide semiconductor filmpreferably includes In, Zn, and M, where M is Ti, Ga, Y, Zr, La, Ce, Nd,Sn, or Hf. Moreover, the oxide semiconductor film preferably includes acrystal part, and the crystal part preferably includes a portion whosec-axis is parallel to a normal vector of a surface over which the oxidesemiconductor film is formed.

With one embodiment of the present invention, a change in electricalcharacteristics can be inhibited and reliability can be improved in asemiconductor device including an oxide semiconductor. With oneembodiment of the present invention, a manufacturing method of asemiconductor device including an oxide semiconductor in which a changein electrical characteristics is inhibited and reliability is improved,can be provided. With one embodiment of the present invention, amanufacturing method of a semiconductor device with low powerconsumption can be provided. With one embodiment of the presentinvention, a novel semiconductor device can be provided. With oneembodiment of the present invention, a method for manufacturing a novelsemiconductor device can be provided. With one embodiment of the presentinvention, a novel display device can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily have all of these effects. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1E are cross-sectional views illustrating a semiconductordevice and an example of a manufacturing process of the semiconductordevice;

FIGS. 2A to 2C are model diagrams used for calculation of movement ofexcess oxygen;

FIGS. 3A to 3C are model diagrams used for calculation of movement ofexcess oxygen;

FIGS. 4A to 4C are model diagrams used for calculation of movement ofexcess oxygen;

FIGS. 5A to 5C are model diagrams used for calculation of movement ofexcess oxygen;

FIG. 6 shows calculation results for demonstrating the movement easinessof excess oxygen;

FIGS. 7A to 7C are model diagrams used for calculation of movement of anoxygen vacancy;

FIGS. 8A to 8C are model diagrams used for calculation of movement of anoxygen vacancy;

FIG. 9 shows calculation results for demonstrating the movement easinessof an oxygen vacancy;

FIGS. 10A and 10B are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device;

FIGS. 11A to 11C are a plan view and cross-sectional views illustratingan example of a semiconductor device;

FIGS. 12A to 12C are a plan view and cross-sectional views illustratingan example of a semiconductor device;

FIGS. 13A to 13C are a plan view and cross-sectional views illustratingan example of a semiconductor device;

FIGS. 14A to 14C are a plan view and cross-sectional views illustratingan example of a semiconductor device;

FIGS. 15A to 15D are cross-sectional views illustrating examples of asemiconductor device;

FIGS. 16A and 16B each show a band structure;

FIGS. 17A to 17D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device;

FIGS. 18A to 18D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device;

FIGS. 19A to 19D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device;

FIGS. 20A and 20B are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device;

FIGS. 21A to 21D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device;

FIGS. 22A to 22D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device;

FIG. 23 shows results of TDS analysis;

FIGS. 24A to 24D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS;

FIGS. 25A to 25D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS;

FIGS. 26A to 26C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD;

FIGS. 27A and 27B show electron diffraction patterns of a CAAC-OS;

FIG. 28 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation;

FIGS. 29A to 29C are a block diagram and circuit diagrams illustrating adisplay device;

FIGS. 30A and 30B are perspective views illustrating an example of atouch panel;

FIGS. 31A and 31B are cross-sectional views illustrating examples of adisplay device;

FIG. 32 is a cross-sectional view illustrating an example of a touchsensor;

FIGS. 33A and 33B are cross-sectional views illustrating examples of atouch panel;

FIGS. 34A and 34B are a block diagram and a timing chart of a touchsensor;

FIG. 35 is a circuit diagram of a touch sensor;

FIG. 36 illustrates a display module;

FIGS. 37A to 37G each illustrate an electronic device;

FIG. 38 shows hydrogen concentration depth profiles of samples inExample;

FIGS. 39A and 39B show electrical characteristics measurement results oftransistors in Example;

FIG. 40 shows electrical characteristics measurement results oftransistors in Example;

FIGS. 41A and 41B show electrical characteristics measurement results oftransistors in Example;

FIG. 42 shows electrical characteristics measurement results oftransistors in Example;

FIGS. 43A and 43B show results of GBT stress tests and ΔVth with respectto stress time of transistors in Example;

FIG. 44 shows oxygen concentration depth profiles of samples inReference Example;

FIGS. 45A and 45B are cross-sectional views illustrating an example of asemiconductor device; and

FIG. 46 is a cross-sectional view illustrating an example of a displaydevice.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.The embodiments can be implemented with various modes, and it will bereadily appreciated by those skilled in the art that modes and detailscan be changed in various ways without departing from the spirit andscope of the present invention. Thus, the present invention should notbe interpreted as being limited to the following description of theembodiments.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Note that the ordinal numbers such as “first”, “second”, and the like inthis specification and the like are used for convenience and do notdenote the order of steps or the stacking order of layers. Therefore,for example, description can be made even when “first” is replaced with“second” or “third”, as appropriate. In addition, the ordinal numbers inthis specification and the like are not necessarily the same as thosewhich specify one embodiment of the present invention.

Note that in this specification, terms for describing arrangement, suchas “over” “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. Further, the positional relation between components is changedas appropriate in accordance with a direction in which the component aredescribed. Thus, the positional relation is not limited to thatdescribed with a term used in this specification and can be explainedwith another term as appropriate depending on the situation.

The “semiconductor device” in this specification and the like means alldevices which can operate by utilizing semiconductor characteristics. Asemiconductor element such as a transistor, a semiconductor circuit, anarithmetic device, and a memory device are each an embodiment of asemiconductor device. An imaging device, a display device, a liquidcrystal display device, a light-emitting device, an electro-opticaldevice, a power generation device (including a thin film solar cell, anorganic thin film solar cell, and the like), and an electronic devicemay include a semiconductor device.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow through thedrain region, the channel region, and the source region. Note that inthis specification and the like, a channel region refers to a regionthrough which current mainly flows.

Further, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an “object having any electric function” aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Examples of an “objecthaving any electric function” are a switching element such as atransistor, a resistor, an inductor, a capacitor, and elements with avariety of functions as well as an electrode and a wiring.

Note that in this specification and the like, a “silicon oxynitridefilm” refers to a film that includes oxygen at a higher proportion thannitrogen, and a “silicon nitride oxide film” refers to a film thatincludes nitrogen at a higher proportion than oxygen.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, the term “conductive layer”can be changed into the term “conductive film” in some cases. Also, theterm “insulating film” can be changed into the term “insulating layer”in some cases.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “substantially parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −30° and lessthan or equal to 30°. Furthermore, the term “perpendicular” indicatesthat the angle formed between two straight lines is greater than orequal to 80° and less than or equal to 100°, and accordingly alsoincludes the case where the angle is greater than or equal to 85° andless than or equal to 95°. The term “substantially perpendicular”indicates that the angle formed between two straight lines is greaterthan or equal to 60° and less than or equal to 120°.

Embodiment 1

In this embodiment, a semiconductor device that is one embodiment of thepresent invention, a manufacturing method of the semiconductor device,and the like are described with reference to FIGS. 1A to 1E to FIGS. 10Aand 10B and FIG. 23.

<1-1. Structural Example of Semiconductor Device>

FIG. 1A is a cross-sectional view illustrating a semiconductor device100 of one embodiment of the present invention. FIGS. 1B to 1E arecross-sectional views illustrating a manufacturing method of thesemiconductor device 100.

The semiconductor device 100 illustrated in FIG. 1A includes an oxidesemiconductor film 108 over a substrate 102, an oxide insulating film114 over the oxide semiconductor film 108, and a protective film 130over the oxide insulating film 114. In this embodiment, the oxideinsulating film 114 includes an oxide insulating film 114 a and an oxideinsulating film 114 b over the oxide insulating film 114 a.

In the step of forming the oxide semiconductor film 108 of thesemiconductor device 100 of one embodiment of the present invention,oxygen vacancies (also referred to as Vo) are formed in the oxidesemiconductor film 108. Then, oxygen is supplied from the oxideinsulating film 114 positioned over the oxide semiconductor film 108, sothat the oxygen vacancies are filled. Note that in the step of formingthe oxide semiconductor film 108, minute defects or lattice defects maybe formed at the same time as the oxygen vacancies in the oxidesemiconductor film 108. Oxygen can be favorably supplied from the oxideinsulating film 114 to the oxide semiconductor film 108 by using theoxygen vacancies, the minute defects, or the lattice defects as adiffusion path of excess oxygen.

Furthermore, in the semiconductor device 100 of one embodiment of thepresent invention, the protective film 130 is formed over the oxideinsulating film 114, and oxygen is added to the oxide insulating film114 through the protective film 130. By adding oxygen to the oxideinsulating film 114 through the protective film 130, the oxideinsulating film 114 can include oxygen more than that in thestoichiometric composition. The oxide insulating film 114 releasesoxygen by being heated. By the release of oxygen, oxygen can be suppliedto the oxide semiconductor film 108.

Thus, in the semiconductor device 100 of one embodiment of the presentinvention, the oxide semiconductor film 108 includes many oxygenvacancies right after it is formed, but can become an oxidesemiconductor film including few oxygen vacancies by the supply ofexcess oxygen from the oxide insulating film 114 to the oxidesemiconductor film 108.

First, each component of the semiconductor device 100 is describedbelow.

(Substrate)

Although there is no particular limitation on a material which can beused as the substrate 102, it is necessary that the material have atleast heat resistance high enough to withstand heat treatment to beperformed later. For example, a glass substrate, a ceramic substrate, aquartz substrate, a sapphire substrate, or the like may be used as thesubstrate 102. Alternatively, a single crystal semiconductor substrateor a polycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like, a compound semiconductor substrate made of silicongermanium or the like, an SOI substrate, or the like may be used as thesubstrate 102. Furthermore, any of these substrates further providedwith a semiconductor element may be used as the substrate 102. In thecase where a glass substrate is used as the substrate 102, a glasssubstrate having any of the following sizes can be used: the 6thgeneration (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm),and the 10th generation (2950 mm×3400 mm). Thus, a large-sized displaydevice can be manufactured.

Still further alternatively, a flexible substrate may be used as thesubstrate 102, and the oxide semiconductor film 108 may be provideddirectly on the flexible substrate. Alternatively, a separation layermay be provided between the substrate 102 and the oxide semiconductorfilm 108. The separation layer can be used when part or the whole of thesemiconductor device 100 formed over the separation layer is separatedfrom the substrate 102 and transferred onto another substrate. In such acase, the semiconductor device 100 can be transferred to a substratehaving low heat resistance or a flexible substrate.

(Oxide Semiconductor Film)

The oxide semiconductor film 108 contains In, Zn, and M (M is Ti, Ga, Y,Zr, La, Ce, Nd, Sn, or Hf). Typically, In—Ga oxide, In—Zn oxide, orIn-M-Zn oxide can be used for the oxide semiconductor film 108. It isparticularly preferable to use In-M-Zn oxide where M is Ga (i.e.,In—Ga—Zn oxide; hereinafter referred to as IGZO) for the oxidesemiconductor film 108.

In the case where the oxide semiconductor film 108 is formed of In-M-Znoxide, it is preferable that the atomic ratio of metal elements of asputtering target used for forming the In-M-Zn oxide satisfy In≥M andZn≥M. As the atomic ratio of metal elements of the sputtering target,In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=2:1:3, In:M:Zn=3:1:2, andIn:M:Zn=4:2:4.1 are preferable. Note that the atomic ratio of metalelements in the formed oxide semiconductor film 108 varies from theabove atomic ratios of metal elements of the sputtering targets in arange of ±40%. For example, when a sputtering target with an atomicratio of In to Ga and Zn of 4:2:4.1 is used, the atomic ratio of In toGa and Zn in the oxide semiconductor film 108 may be 4:2:3 or in thevicinity of 4:2:3.

Note that in the case where the oxide semiconductor film 108 is anIn-M-Zn oxide film, the proportion of In and the proportion of M, nottaking Zn and O into consideration, are preferably greater than 25atomic % and less than 75 atomic %, respectively, and further preferablygreater than 34 atomic % and less than 66 atomic %, respectively.

The energy gap of the oxide semiconductor film 108 is 2 eV or more,preferably 2.5 eV or more and further preferably 3 eV or more.

The thickness of the oxide semiconductor film 108 is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm and further preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 108. For example, an oxide semiconductor filmwhose carrier density is lower than or equal to 1×10¹⁷/cm³, preferablylower than or equal to 1×10¹⁵/cm³, further preferably lower than orequal to 1×10¹³/cm³, and still further preferably lower than or equal to1×10¹¹/cm³ is used as the oxide semiconductor film 108.

The oxide semiconductor film 108 may have a non-single-crystalstructure, for example. The non-single-crystal structure includes ac-axis aligned crystalline oxide semiconductor (CAAC-OS) which isdescribed later, a polycrystalline structure, a microcrystallinestructure described later, or an amorphous structure, for example. Amongthe non-single-crystal structures, the amorphous structure has thehighest density of defect states, whereas CAAC-OS has the lowest densityof defect states.

The oxide semiconductor film 108 may have an amorphous structure, forexample. The oxide semiconductor film having the amorphous structure hasdisordered atomic arrangement and no crystalline component, for example.Alternatively, the oxide film having an amorphous structure has, forexample, an absolutely amorphous structure and no crystal part.

Note that the oxide semiconductor film 108 may be a mixed film includingtwo or more of the following regions: a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle crystal structure. The mixed film may have a single-layerstructure including, for example, two or more of the following regions:a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,a CAAC-OS region, and a region having a single crystal structure. Themixed film may have a stacked-layer structure including, for example,two or more of the following regions: a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle crystal structure.

(Oxide Insulating Film)

The oxide insulating film 114 has a function of supplying oxygen to theoxide semiconductor film 108.

The oxide insulating film 114 can have a structure including oxygen andsilicon. For example, a silicon oxide film or a silicon oxynitride filmcan be used as the oxide insulating film 114.

The oxide insulating film 114 may have a single-layer structure or astacked-layer structure of two or more layers. In this embodiment, theoxide insulating film 114 having a stacked-layer structure of two layerswhich are the oxide insulating film 114 a and the oxide insulating film114 b is described.

As the oxide insulating film 114 a, a silicon oxide film, a siliconoxynitride film, or the like with a thickness greater than or equal to 5nm and less than or equal to 150 nm, preferably greater than or equal to5 nm and less than or equal to 50 nm can be used.

Further, it is preferable that the number of defects in the oxideinsulating film 114 a be small and typically, the spin densitycorresponding to a signal that appears at g=2.001 due to a dangling bondof silicon be lower than or equal to 3×10¹⁷ spins/cm³ by electron spinresonance (ESR) measurement. This is because if the density of defectsin the oxide insulating film 114 a is high, oxygen is bonded to thedefects and the amount of oxygen that passes through the oxideinsulating film 114 a is decreased.

Note that all oxygen entering the oxide insulating film 114 a from theoutside does not move to the outside of the oxide insulating film 114 aand some oxygen remains in the oxide insulating film 114 a. Furthermore,movement of oxygen occurs in the oxide insulating film 114 a in somecases in such a manner that oxygen enters the oxide insulating film 114a and oxygen contained in the oxide insulating film 114 a is moved tothe outside of the oxide insulating film 114 a. When the oxideinsulating film through which oxygen can pass is formed as the oxideinsulating film 114 a, oxygen released from the oxide insulating film114 b provided over the oxide insulating film 114 a can be moved to theoxide semiconductor film 108 through the oxide insulating film 114 a.

Note that the oxide insulating film 114 a can be formed using an oxideinsulating film having a low density of states due to nitrogen oxide.Note that the density of states due to nitrogen oxide can be formedbetween the energy of the valence band maximum (E_(v) _(_) _(os)) andthe energy of the conduction band minimum (E_(c) _(_) _(os)) of theoxide semiconductor film. A silicon oxynitride film that releases littlenitrogen oxide, an aluminum oxynitride film that releases littlenitrogen oxide, or the like can be used as the above oxide insulatingfilm.

Note that a silicon oxynitride film that releases little nitrogen oxideis a film of which the amount of released ammonia is larger than theamount of released nitrogen oxide in thermal desorption spectroscopy(TDS) analysis; the amount of released ammonia is typically greater thanor equal to 1×10¹⁸/cm³ and less than or equal to 5×10¹⁹/cm³. Note thatthe amount of released ammonia is the amount of ammonia released by heattreatment with which the surface temperature of a film becomes higherthan or equal to 50° C. and lower than or equal to 650° C., preferablyhigher than or equal to 50° C. and lower than or equal to 550° C.

The oxide insulating film 114 b is formed using an oxide insulating filmthat includes oxygen more than that in the stoichiometric composition.Part of oxygen is released by heating from the oxide insulating filmincluding oxygen more than that in the stoichiometric composition. Theoxide insulating film including oxygen more than that in thestoichiometric composition is an oxide insulating film of which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁹ atoms/cm³, preferably greater than or equal to3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the temperature of thefilm surface in the TDS analysis is preferably higher than or equal to100° C. and lower than or equal to 700° C., or higher than or equal to100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used as the oxide insulating film 114 b.

It is preferable that the number of defects in the oxide insulating film114 b be small, and typically the spin density corresponding to a signalwhich appears at g=2.001 due to a dangling bond of silicon be lower than1.5×10¹⁸ spins/cm³, preferably lower than or equal to 1×10¹⁸ spins/cm³by ESR measurement. Note that the oxide insulating film 114 b isprovided more apart from the oxide semiconductor film 108 than the oxideinsulating film 114 a is; thus, the oxide insulating film 114 b may havehigher density of defects than the oxide insulating film 114 a.

Furthermore, the oxide insulating films 114 a and 114 b can be formedusing insulating films formed of the same kinds of materials; thus, aboundary between the oxide insulating films 114 a and 114 b cannot beclearly observed in some cases. Although a two-layer structure of theoxide insulating films 114 a and 114 b is described in this embodiment,the present invention is not limited to this. For example, asingle-layer structure of the oxide insulating film 114 a or the oxideinsulating film 114 b may be employed.

(Protective Film)

The protective film 130 has a function of inhibiting release of oxygenfrom the oxide insulating film 114. The protective film 130 includes atleast one element selected from In, Zn, Ga, Sn, Ti, Al, W, Ta, Mo, andSi. For example, the protective film 130 can be formed of an oxide or anitride of the above-described element. It is preferable that theprotective film 130 be formed of an oxide including In, Sn, and Si (anIn—Sn—Si oxide: also referred to as ITSO) because the protective film130 can transmit oxygen at the time of adding oxygen and inhibit releaseof oxygen after the oxygen addition.

<1-2. Manufacturing Method of Semiconductor Device>

Next, the manufacturing method of the semiconductor device 100 isdescribed with reference to FIGS. 1B to 1E.

(1. Step of Forming an Oxide Semiconductor Film)

The oxide semiconductor film 108 is formed over the substrate 102 (seeFIG. 1B).

In this embodiment, the oxide semiconductor film 108 is formed with asputtering apparatus. An In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1in an atomic ratio) is provided in the sputtering apparatus. The oxidesemiconductor film 108 is formed under conditions where oxygen vacanciesare formed.

In the case where the oxide semiconductor film 108 is formed with thesputtering apparatus, a rare gas (typically argon), oxygen, or a mixedgas of a rare gas and oxygen is used as a sputtering gas. In the case ofusing the mixed gas, the proportion of oxygen to a rare gas ispreferably decreased to increase oxygen vacancies in the oxidesemiconductor film 108. For example, when the partial pressure of oxygenin the sputtering apparatus is higher than 0% and lower than 50%, oxygenvacancies in the oxide semiconductor film 108 can be increased. Notethat in the case where the partial pressure of oxygen in the sputteringapparatus is 0%, oxygen vacancies are excessively increased in the oxidesemiconductor film 108 or the film quality of the oxide semiconductorfilm 108 is changed, and accordingly, in some cases, the oxygenvacancies cannot be filled or the resistance of the oxide semiconductorfilm 108 is not decreased by the later addition of excess oxygen to theoxide semiconductor film 108. Therefore, it is preferable to set thepartial pressure of oxygen in formation of the oxide semiconductor film108 to be at least higher than 0%. Furthermore, in the case of formingthe oxide semiconductor film 108, the partial pressure of oxygen in thesputtering apparatus may be higher than or equal to 50% and lower thanor equal to 100%. In this case, the number of oxygen vacancies is smallin the oxide semiconductor film 108; therefore, a heating step may beperformed in a period after the oxide semiconductor film 108 is formedand before the oxide insulating film 114 is formed to increase oxygenvacancies in the oxide semiconductor film 108.

In this embodiment, the oxide semiconductor film 108 is formed at anoxygen partial pressure of 30% in the sputtering apparatus.

In the formation of the oxide semiconductor film 108 with the sputteringapparatus, increasing the purity of the sputtering gas is necessary. Forexample, an oxygen gas or an argon gas used for a sputtering gas ishighly purified to have a dew point of −40° C. or lower, preferably −80°C. or lower, further preferably −100° C. or lower, still furtherpreferably −120° C. or lower, whereby entry of moisture or the like intothe oxide semiconductor film 108 can be minimized. Furthermore, achamber in the sputtering apparatus is preferably subjected to highvacuum evacuation (to a vacuum of about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with useof a suction vacuum evacuation pump such as a cryopump so that water orthe like, which is an impurity for the oxide semiconductor film 108, isremoved as much as possible. Alternatively, a turbo molecular pump and acold trap are preferably combined so as to prevent a backflow of a gas,especially a gas containing carbon or hydrogen from an exhaust system tothe inside of the chamber.

In the case of performing the heating step after the oxide semiconductorfilm 108 is formed, the heating step may be performed at a temperaturehigher than or equal to 150° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 200° C. and lower than orequal to 450° C., or further preferably higher than or equal to 300° C.and lower than or equal to 450° C. The heating step performed hereserves as one kind of treatment for increasing the purity of the oxidesemiconductor film and can reduce hydrogen, water, and the likecontained in the oxide semiconductor film 108. In addition, the heatingstep is one kind of treatment for increasing oxygen vacancies in theoxide semiconductor film 108.

An electric furnace, an RTA apparatus, or the like can be used for theheating step performed on the oxide semiconductor film 108. With the useof an RTA apparatus, heat treatment can be performed at a temperaturehigher than or equal to the strain point of the substrate if the heatingtime is short. Therefore, the heat treatment time can be shortened.

In the case of performing the heating step after the oxide semiconductorfilm 108 is formed, the heating step may be performed under anatmosphere of nitrogen, ultra-dry air (air in which a water content is20 ppm or less, preferably 1 ppm or less, further preferably 10 ppb orless), or a rare gas (argon, helium, or the like). In the case ofperforming the heating step after the oxide semiconductor film 108 isformed, the heating step may be performed in a reduced-pressureatmosphere (an atmosphere at a pressure lower than the atmosphericpressure). In the case of performing the heating step after the oxidesemiconductor film 108 is formed, the heating step is preferablyperformed under an atmosphere with an oxygen partial pressure of 1% orlower. For example, in the case of performing the heating step after theoxide semiconductor film 108 is formed, if the heating step is performedunder an atmosphere with an oxygen partial pressure of higher than 1%,oxygen might be supplied to the oxide semiconductor film 108 beforeformation of the oxide insulating film 114 to fill oxygen vacancies inthe oxide semiconductor film 108.

It should be noted that oxygen vacancies in the oxide semiconductor film108 are preferably filled with excess oxygen from the oxide insulatingfilm 114. For example, in the case of performing the heating step underan atmosphere with an oxygen partial pressure of higher than 1% afterthe oxide semiconductor film 108 is formed, oxygen vacancies in thevicinity of the surface of the oxide semiconductor film 108 might befilled but oxygen vacancies in the oxide semiconductor film 108 or inthe vicinity of a formation surface of the oxide semiconductor film 108might not be filled.

Furthermore, in the case where the oxide semiconductor film 108 is acrystalline oxide semiconductor film such as a CAAC-OS film to bedescribed later, a plurality of In—O layers overlapping in the c-axisdirection might inhibit the passing of excess oxygen. In this case,plasma treatment or the like may be performed on the surface and itsvicinity of the oxide semiconductor film 108 to lower the crystallinityof the surface and its vicinity of the oxide semiconductor film 108,that is, to increase minute defects or lattice defects in the surface orits vicinity of the oxide semiconductor film 108. The increase in minutedefects or lattice defects in the surface or its vicinity of the oxidesemiconductor film 108 enables favorable addition of excess oxygen tothe oxide semiconductor film via the minute defects or the latticedefects or favorable diffusion of oxygen toward the bottom region of theoxide semiconductor film 108 at the time of supplying oxygen from theoxide insulating film 114 that is in contact with the upper surface sideof the oxide semiconductor film 108.

(2. Step of Forming an Oxide Insulating Film)

Next, the oxide insulating film 114 is formed over the oxidesemiconductor film 108 (see FIG. 1C).

In this embodiment, as the oxide insulating film 114 a and the oxideinsulating film 114 b, silicon oxynitride films are used.

After the oxide insulating film 114 a is formed, the oxide insulatingfilm 114 b is preferably formed in succession without exposure to theair. When the oxide insulating film 114 b is formed by adjusting atleast one of the flow rate of a source gas, pressure, a high-frequencypower, and a substrate temperature without exposure to the air after theoxide insulating film 114 a is formed, the concentration of impuritiesattributed to the atmospheric component at the interface between theoxide insulating film 114 a and the oxide insulating film 114 b can bereduced.

For example, in the case where a silicon oxynitride film is formed asthe oxide insulating film 114 a by a PECVD method, a deposition gascontaining silicon and an oxidizing gas are preferably used as a sourcegas. Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride. Examples of theoxidizing gas include dinitrogen monoxide and nitrogen dioxide. Theoxide insulating film 114 a can include nitrogen and have a small numberof defects by setting the ratio of the oxidizing gas to the depositiongas to be higher than 20 times and lower than 100 times, preferablyhigher than or equal to 40 times and lower than or equal to 80 times andthe pressure in a treatment chamber to be lower than 100 Pa, preferablylower than or equal to 50 Pa in the PECVD method.

In this embodiment, a silicon oxynitride film is formed as the oxideinsulating film 114 a by a PECVD method under the conditions where thesubstrate 102 is held at a temperature of 220° C., silane at a flow rateof 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm are usedas a source gas, the pressure in the treatment chamber is 20 Pa, and ahigh-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as the powerdensity) is supplied to parallel-plate electrodes.

As the oxide insulating film 114 b, a silicon oxynitride film is formedunder the conditions where the substrate placed in a treatment chamberof the PECVD apparatus, which is vacuum-evacuated, is held at atemperature higher than or equal to 180° C. and lower than or equal to280° C., preferably higher than or equal to 200° C. and lower than orequal to 240° C., the pressure in the treatment chamber is greater thanor equal to 100 Pa and less than or equal to 250 Pa, preferably greaterthan or equal to 100 Pa and less than or equal to 200 Pa withintroduction of a source gas into the treatment chamber, and ahigh-frequency power greater than or equal to 0.17 W/cm² and less thanor equal to 0.5 W/cm², preferably greater than or equal to 0.25 W/cm²and less than or equal to 0.35 W/cm², is supplied to an electrodeprovided in the treatment chamber.

As the film formation conditions of the oxide insulating film 114 b, thehigh-frequency power having the above power density is supplied to areaction chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; thus, the oxygencontent in the oxide insulating film 114 b becomes higher than that inthe stoichiometric composition. In the film formed at a substratetemperature within the above temperature range, however, the bondbetween silicon and oxygen is weak, and part of oxygen in the film isreleased by heat treatment in a later step. Thus, it is possible to forman oxide insulating film which includes oxygen more than that in thestoichiometric composition and from which part of oxygen is released byheating.

Note that in the deposition conditions of the oxide insulating film 114b, when the flow rate of the deposition gas containing silicon withrespect to the oxidizing gas is increased, the amount of defects in theoxide insulating film 114 b can be reduced. Typically, it is possible toform an oxide insulating film in which the number of defects is small,that is, the spin density corresponding to a signal which appears atg=2.001 due to a dangling bond of silicon is lower than 6×10¹⁷spins/cm³, preferably lower than or equal to 3×10¹⁷ spins/cm³, andfurther preferably lower than or equal to 1.5×10¹⁷ spins/cm³, by ESRmeasurement.

After the step of forming the oxide insulating film 114, a step ofheating the oxide insulating film 114 may be performed. In this case,impurities such as hydrogen or water contained in the oxide insulatingfilm 114 can be removed.

The temperature of the step of heating the oxide insulating film 114 istypically higher than or equal to 150° C. and lower than or equal to400° C., preferably higher than or equal to 300° C. and lower than orequal to 400° C., further preferably higher than or equal to 320° C. andlower than or equal to 370° C. The heat treatment may be performed underan atmosphere of nitrogen, oxygen, ultra-dry air (air in which a watercontent is 20 ppm or less, preferably 1 ppm or less, further preferably10 ppb or less), or a rare gas (argon, helium, or the like). Note thatan electric furnace, an RTA apparatus, or the like can be used for theheat treatment during which it is preferable that hydrogen, water, andthe like not be contained in the nitrogen, oxygen, ultra-dry air, orrare gas.

(3. Step of Forming a Protective Film)

Next, the protective film 130 is formed over the oxide insulating film114 (see FIG. 1D).

In this embodiment, an ITSO film is used as the protective film 130. TheITSO film used as the protective film 130 is formed to have a thicknessof 5 nm with a sputtering apparatus. Note that the thickness of theprotective film 130 is preferably greater than or equal to 1 nm and lessthan or equal to 20 nm or greater than or equal to 2 nm and less than orequal to 10 nm, in which case oxygen is favorably transmitted andrelease of oxygen can be inhibited.

(4. Step of Adding Oxygen to the Oxide Insulating Film)

Next, oxygen 142 is added to the oxide insulating film 114 through theprotective film 130 (see FIG. 1E).

As a method for adding the oxygen 142 to the oxide insulating film 114through the protective film 130, there are an ion doping method, an ionimplantation method, plasma treatment, and the like. A plasma apparatusis preferably used for the method for adding the oxygen 142 because alarge amount of oxygen can be added to the oxide insulating film 114. Byadding the oxygen 142 to the oxide insulating film 114 through theprotective film 130, the oxygen 142 may be added to the oxidesemiconductor film 108.

By application of bias voltage to the substrate 102 side when the oxygen142 is added, the oxygen 142 can be effectively added to the oxideinsulating film 114. The bias voltage is applied, for example, by anashing apparatus with the power density applied to the substrate side ofthe ashing apparatus be set to be greater than or equal to 1 W/cm² andless than or equal to 5 W/cm². When the substrate temperature duringaddition of the oxygen 142 is higher than or equal to room temperatureand lower than or equal to 300° C., preferably higher than or equal to100° C. and lower than or equal to 250° C., the oxygen can be addedefficiently to the oxide insulating film 114. In the case where oxygenis introduced by plasma treatment, oxygen may be made excited by amicrowave to generate high density oxygen plasma, so that the amount ofoxygen introduced into the oxide insulating film 114 can be increased.

When the protective film 130 is provided over the oxide insulating film114 and then the oxygen 142 is added, the protective film 130 functionsas a protective film for inhibiting release of oxygen from the oxideinsulating film 114. In this way, a large amount of oxygen can be addedto the oxide insulating film 114.

(5. Step of Filling Oxygen Vacancies in the Oxide Semiconductor Film)

Next, oxygen vacancies in the oxide semiconductor film 108 are filledwith oxygen from the oxide insulating film 114.

For example, at the time of adding the oxygen 142 in (4. Step of addingoxygen to the oxide insulating film), oxygen from the oxide insulatingfilm 114 and the oxygen 142 fill oxygen vacancies in the oxidesemiconductor film 108. Furthermore, for example, heat treatment isperformed after (4. Step of adding oxygen to the oxide insulating film)so that oxygen from the oxide insulating film 114 (oxygen constitutingpart of the oxide insulating film 114 or oxygen added in the step ofadding oxygen) can fill oxygen vacancies in the oxide semiconductor film108. The heat treatment may be performed at a temperature higher than orequal to 100° C. and lower than or equal to 600° C., preferably higherthan or equal to 100° C. and lower than or equal to 350° C., furtherpreferably higher than or equal to 150° C. and lower than or equal to350° C.

As described above, when forming the oxide semiconductor film 108 of thesemiconductor device 100 of one embodiment of the present invention,oxygen vacancies are formed and then filled with oxygen from the oxideinsulating film 114. The oxygen vacancies can serve as a diffusion pathof oxygen for diffusing oxygen from the oxide insulating film 114.Accordingly, oxygen can be efficiently diffused from the oxideinsulating film 114 compared with the case in which oxygen is diffusedin the state where the oxide semiconductor film 108 includes a smallnumber of oxygen vacancies. As a result, the oxide semiconductor film108 including a small number of oxygen vacancies can be formed.

<1-3. Oxygen-Transmitting Property of Oxide Semiconductor Film>

Here, the oxygen-transmitting property of the oxide semiconductor film108 influenced by formation conditions of the oxide semiconductor film108 is described with reference to FIG. 23.

Samples described below were formed in order to evaluate theoxygen-transmitting property of the oxide semiconductor film 108.

A silicon oxynitride film with a thickness of 400 nm functioning as theoxide insulating film 114 was formed over a substrate. The siliconoxynitride film was deposited under the conditions where the substratetemperature was 220° C., a silane gas at a flow rate of 50 sccm and adinitrogen monoxide gas at a flow rate of 2000 sccm were introduced intoa chamber, the pressure was 20 Pa, and an RF power of 100 W was suppliedbetween parallel-plate electrodes provided in a PECVD apparatus.

Next, heat treatment was performed. The heat treatment was performed at350° C. in a nitrogen gas atmosphere for 1 hour.

Then, an ITSO film with a thickness of 5 nm functioning as theprotective film 130 was formed over the silicon oxynitride film. TheITSO film was formed with a sputtering apparatus. Note that thecomposition of a target used for forming the ITSO film wasIn₂O₃:SnO₂:SiO₂=85:10:5 [wt %].

Next, oxygen addition treatment was performed through the ITSO film. Theoxygen addition treatment was performed with an ashing apparatus underthe conditions where an oxygen gas at a flow rate of 250 sccm wasintroduced into a chamber, the pressure was 15 Pa, and an RF power of4500 W was supplied between parallel-plate electrodes provided in theashing apparatus so that a bias can be applied to the substrate side.

Then, the ITSO film was removed by etching with a wet etching apparatuswith the use of a 5% oxalic acid solution for 300 sec.

Next, an IGZO film with a thickness of 10 nm functioning as the oxidesemiconductor film 108 was formed over the silicon oxynitride film. TheIGZO film was formed by a sputtering method using a polycrystallinetarget of a metal oxide with the following atomic ratio,In:Ga:Zn=4:2:4.1.

For the formation of the IGZO film, five different conditions of theoxygen partial pressure were set as follows: 10%, 20%, 30%, 40%, and50%.

Then, TDS analysis was performed on the samples formed under the fiveconditions. FIG. 23 shows the results of the TDS analysis. In FIG. 23,the vertical axis shows the intensity of ions with a mass-to-chargeratio (m/z) of 32 emitted from each sample, that is, the intensity ofoxygen, and the horizontal axis shows the temperature.

As shown in FIG. 23, the amount of emitted oxygen in the case where theoxygen partial pressure for forming the IGZO film was 10%, 20%, 30%, and40% was larger than that in the case where the oxygen partial pressurewas 50% in the TDS analysis. In other words, by setting the oxygenpartial pressure for forming the IGZO film to be higher than 0% andlower than 50%, the oxygen-transmitting property can become higher thanthat in the case where the oxygen partial pressure is higher than orequal to 50% and less than or equal to 100%. Furthermore, as shown inFIG. 23, the amount of emitted oxygen in the TDS analysis was higher inthe order of the samples formed under the oxygen partial pressures of10%, 20%, 30%, 40%, and 50%. Note that the amount of emitted oxygenshown in FIG. 23 includes both the amount of emitted oxygen from theoxide insulating film 114 and the amount of emitted oxygen from theoxide semiconductor film 108. In consideration of the oxygen content inthe oxide insulating film 114 and the oxide semiconductor film 108,however, the emitted oxygen shown in FIG. 23 is mostly oxygen emittedfrom the oxide insulating film 114 and released through the oxidesemiconductor film 108.

Thus, by varying the oxygen partial pressure for forming the oxidesemiconductor film 108, the oxygen-transmitting property of the oxidesemiconductor film 108 can be controlled. The above results suggest thata high oxygen-transmitting property can be obtained when the oxidesemiconductor film 108 include many oxygen vacancies, many minutedefects, or many lattice defects. In other words, theoxygen-transmitting property can be increased by forming many oxygenvacancies, minute defects, or lattice defects in the oxide semiconductorfilm 108 so that the oxygen vacancies or defects can serve as adiffusion path of oxygen.

<1-4. Concept of the Oxygen Supply to Oxide Semiconductor Film>

Next, the concept of the oxygen supply to the oxide semiconductor film108 is described below based on the model diagrams illustrated in FIGS.2A to 2C, FIGS. 3A to 3C, FIGS. 4A to 4C, FIGS. 5A to 5C, FIGS. 7A to7C, FIGS. 8A to 8C and calculation results shown in FIG. 6 and FIG. 9.

Here, taking an IGZO film as an example of the oxide semiconductor film108, the movement easiness of excess oxygen (oxygen more than that inthe stoichiometric composition) and oxygen vacancies are described.

In this embodiment, models in which either one excess oxygen atom or oneoxygen vacancy existed on one In—O plane of an IGZO film with an atomicratio of In:Ga:Zn=3:1:2 were made by geometry optimization, and energycorresponding to an intermediate structure along a minimum energy pathin each model was calculated by a nudged elastic band (NEB) method.

The calculation was performed using calculation program software“OpenMX” based on the density functional theory (DFT). As a basisfunction used as a parameter in the calculation, a pseudo-atomiclocalized basis function was used. Note that the basis function iscategorized into polarized basis sets of slater type orbital (STO). As afunctional, generalized-gradient-approximation/Perdew-Burke-Ernzerhof(GGA/PBE) was used. The cut-off energy was set to 200 Ry. The number ofsampling k points was 5×5×3.

In the calculation of the movement easiness of excess oxygen, the numberof atoms which existed in the calculation model was set to 85. In thecalculation of the movement easiness of an oxygen vacancy, the number ofatoms which existed in the calculation model was set to 83.

The movement easiness of excess oxygen and the movement easiness of anoxygen vacancy were evaluated by calculation of a height of energybarrier Eb which is required for excess oxygen or an oxygen vacancy togo over in moving to respective sites. That is, when the height ofenergy barrier Eb which excess oxygen or an oxygen vacancy goes over ishigh, the movement of the excess oxygen or the oxygen vacancy isdifficult, and when the height of the energy barrier Eb is low, themovement of the excess oxygen or the oxygen vacancy is easy.

(Movement of Excess Oxygen)

First, the movement of excess oxygen is described. The models in whichone excess oxygen atom exists in one In—O plane of the IGZO film with anatomic ratio of In:Ga:Zn=3:1:2 are shown in FIGS. 2A to 2C, FIGS. 3A to3C, FIGS. 4A to 4C, and FIGS. 5A to 5C.

((1) First Transition of Excess Oxygen)

FIG. 2A is a model diagram of the IGZO film, FIG. 2B is an enlargedmodel diagram showing a region a1 in FIG. 2A, and FIG. 2C is a modeldiagram showing the transition of excess oxygen from the site shown inthe model diagram of FIG. 2B. The transition from FIG. 2B to FIG. 2C isreferred to as a first transition of excess oxygen. In the firsttransition of excess oxygen, excess oxygen diffuses from an InO₂ layerto a (Ga, Zn)O layer.

((2) Second Transition of Excess Oxygen)

FIG. 3A is a model diagram of the IGZO film, FIG. 3B is an enlargedmodel diagram showing a region a2 in FIG. 3A, and FIG. 3C is a modeldiagram showing the transition of excess oxygen from the site shown inthe model diagram of FIG. 3B. The transition from FIG. 3B to FIG. 3C isreferred to as a second transition of excess oxygen. In the secondtransition of excess oxygen, excess oxygen diffuses from a first (Ga,Zn)O layer to a second (Ga, Zn)O layer.

((3) Third Transition of Excess Oxygen)

FIG. 4A is a model diagram of the IGZO film, FIG. 4B is an enlargedmodel diagram showing a region a3 in FIG. 4A, and FIG. 4C is a modeldiagram showing the transition of excess oxygen from the site shown inthe model diagram of FIG. 4B. The transition from FIG. 4B to FIG. 4C isreferred to as a third transition of excess oxygen. In the thirdtransition of excess oxygen, excess oxygen diffuses along an In layer.

((4) Fourth Transition of Excess Oxygen)

FIG. 5A is a model diagram of the IGZO film, FIG. 5B is an enlargedmodel diagram showing a region a4 in FIG. 5A, and FIG. 5C is a modeldiagram showing the transition of excess oxygen from the site shown inthe model diagram of FIG. 5B. The transition from FIG. 5B to FIG. 5C isreferred to as a fourth transition of excess oxygen. In the fourthtransition of excess oxygen, excess oxygen diffuses by moving across anIn layer.

In FIGS. 2B and 2C, FIGS. 4B and 4C, and FIGS. 5B and 5C, an oxygen atomnumbered “1” is referred to as a first oxygen atom. In FIGS. 2B and 2C,FIGS. 4B and 4C, and FIGS. 5B and 5C, an oxygen atom numbered “2” isreferred to as a second oxygen atom. In FIGS. 3B and 3C, FIGS. 4B and4C, and FIGS. 5B and 5C, an oxygen atom numbered “3” is referred to as athird oxygen atom. In FIGS. 3B and 3C, an oxygen atom numbered “4” isreferred to as a fourth oxygen atom.

FIG. 6 shows calculation results showing the movement easiness of excessoxygen in the cases of the above-described four transition patterns. InFIG. 6, the horizontal axis represents the path length of the excessoxygen's movement, and the vertical axis represents energy required forthe movement relative to the energy in the state shown in FIGS. 2B, 3B,4B, and 5B.

As shown in FIG. 6, the maximum height of the energy barrier Eb(Eb_(max)) in the first transition of excess oxygen is 0.62 eV, and themaximum height of the energy barrier Eb (Eb_(max)) in the secondtransition of excess oxygen is 0.29 eV. The maximum height of the energybarrier Eb (Eb_(max)) in the third transition of excess oxygen is 0.53eV, and the maximum height of the energy barrier Eb (Eb_(max)) in thefourth transition of excess oxygen is 2.38 eV. Thus, the maximum heightsof the energy barrier Eb (Eb_(max)) in the first to third transitions ofexcess oxygen are lower than that in the fourth transition of excessoxygen. This means that the energies required for the first to thirdtransitions of excess oxygen are lower than that required for the fourthtransition of excess oxygen and the first to third transitions of excessoxygen are more likely to occur than the fourth transition of excessoxygen does.

In other words, the first oxygen atom in the model shown in FIG. 2B,FIG. 4B, and FIG. 5B is more likely to move in the direction of pushingout the second oxygen atom shown in FIGS. 2B and 2C and FIGS. 4B and 4Cthan in the direction of pushing out the third oxygen atom shown inFIGS. 5B and 5C.

The third oxygen atom in the model shown in FIG. 3B is likely to move inthe direction of pushing out the fourth oxygen atom shown in FIG. 3C.This shows that an oxygen atom moves along the layer of indium atomsmore easily than across the layer of indium atoms. Further, an oxygenatom moves from the InO₂ layer to the (Ga, Zn)O layer and from the first(Ga, Zn)O layer to the second (Ga, Zn)O layer more easily than acrossthe layer of indium atoms.

(Movement of Oxygen Vacancy)

Next, the movement of an oxygen vacancy is described. The models inwhich one oxygen vacancy exists in one In—O plane of an IGZO film withan atomic ratio of In:Ga:Zn=3:1:2 are shown in FIGS. 7A to 7C and FIGS.8A to 8C.

((5) First Transition of Oxygen Vacancy)

FIG. 7A is a model diagram of the IGZO film, FIG. 7B is an enlargedmodel diagram showing a region a5 in FIG. 7A, and FIG. 7C is a modeldiagram showing the transition of an oxygen vacancy from the site shownin the model diagram of FIG. 7B. The transition from FIG. 7B to FIG. 7Cis referred to as a first transition of an oxygen vacancy. In the firsttransition of an oxygen vacancy, an oxygen vacancy diffuses along an Inlayer.

((6) Second Transition of Oxygen Vacancy)

FIG. 8A is a model diagram of the IGZO film, FIG. 8B is an enlargedmodel diagram showing a region a6 in FIG. 8A, and FIG. 8C is a modeldiagram showing the transition of an oxygen vacancy from the site shownin the model diagram of FIG. 8B. The transition from FIG. 8B to FIG. 8Cis referred to as a second transition of an oxygen vacancy. In thesecond transition of an oxygen vacancy, an oxygen vacancy diffuses bymoving across an In layer.

In FIGS. 7B and 7C and FIGS. 8B and 8C, a dotted circle represents anoxygen vacancy.

FIG. 9 shows calculation results showing the movement easiness of anoxygen vacancy in the cases of the above-described two transitionpatterns. In FIG. 9, the horizontal axis represents the path length ofthe oxygen vacancy's movement, and the vertical axis represents energyrequired for the movement relative to the energy in the state shown inFIGS. 7B and 8B.

As shown in FIG. 9, the maximum height of the energy barrier Eb(Eb_(max)) in the first transition of an oxygen vacancy is 1.81 eV, andthe maximum height of the energy barrier Eb (Eb_(max)) in the secondtransition of an oxygen vacancy is 4.10 eV. The maximum height of theenergy barrier Eb (Eb_(max)) in the first transition of an oxygenvacancy is lower than that in the second transition of an oxygenvacancy. This means that the energy required for the first transition ofan oxygen vacancy is lower than that required for the second transitionof an oxygen vacancy. In other words, the first transition of an oxygenvacancy is more likely to occur than the second transition of an oxygenvacancy does.

This shows that like the movement of excess oxygen described above, anoxygen vacancy moves along the layer of indium atoms more easily thanacross the layer of indium atoms.

(Temperature Dependence of Transition)

Next, in order to compare probabilities of occurrence of theabove-described six transition patterns from another perspective,temperature dependence of these transitions is described below.

Temperature dependence of these transitions is compared based on themovement frequency per unit time. Here, the movement frequency Z (timesper second) at certain temperature is represented by Formula (1) usingthe number of vibrations Zo (times per second) of an oxygen atom in aposition where the oxygen atom is chemically stable.

$\begin{matrix}{Z = {{Zo} \cdot {\exp \left( {- \frac{{Eb}_{\max}}{kT}} \right)}}} & (1)\end{matrix}$

Note that in Formula 1, Eb_(max) represents the maximum height of theenergy barrier Eb in each transition, k represents Boltzmann constant, Trepresents the absolute temperature, and Zo represents the number ofvibrations of an atom in a stable position. In this embodiment,calculation is performed on the assumption that Zo is 1.0×10¹³ (timesper second), which is a typical Debye frequency.

Z has the following values when T is 300 K (27° C.).

-   (1) First transition of excess oxygen Z=3.9×10² (times per second)    at T=300 K.-   (2) Second transition of excess oxygen Z=1.2×10⁸ (times per second)    at T=300 K.-   (3) Third transition of excess oxygen Z=1.2×10⁴ (times per second)    at T=300 K.-   (4) Fourth transition of excess oxygen Z=1.0×10⁻²⁷ (times per    second) at T=300 K.-   (5) First transition of oxygen vacancy Z=4.3×10⁻¹⁸ (times per    second) at T=300 K.-   (6) Second transition of oxygen vacancy Z=1.4×10⁻⁵⁶ (times per    second) at T=300 K.

Furthermore, Z has the following values when T is 723 K (450° C.).

-   (1) First transition of excess oxygen Z=4.8×10⁸ (times per second)    at T=723 K.-   (2) Second transition of excess oxygen Z=9.2×10¹⁰ (times per second)    at T=723 K.-   (3) Third transition of excess oxygen Z=2.0×10⁹ (times per second)    at T=723 K.-   (4) Fourth transition of excess oxygen Z=2.5×10⁻⁴ (times per second)    at T=723 K.-   (5) First transition of oxygen vacancy Z=2.5 (times per second) at    T=723 K.-   (6) Second transition of oxygen vacancy Z=2.5×10⁻¹⁶ (times per    second) at T=723 K.

In view of the above-described calculation results, excess oxygen, inthe case of either T=300 K or T=723 K, moves along the layer of indiumatoms more easily than across the layer of indium atoms. Moreover, anoxygen vacancy also, in the case where either T=300 K or T=723 K, movesalong the layer of indium atoms more easily than across the layer ofindium atoms.

At T=300 K, the movement of excess oxygen along the layer of indiumatoms, the movement of excess oxygen from the InO₂ layer to the (Ga,Zn)O layer, and the movement of excess oxygen from the first (Ga, Zn)Olayer to the second (Ga, Zn)O layer are likely to occur; in contrast,the other transitions are unlikely to occur. At T=723 K, the movement ofan oxygen vacancy along the layer of indium atoms as well as theabove-mentioned movements of excess oxygen is likely to occur; incontrast, the movements of both excess oxygen and an oxygen vacancyacross the layer of indium atoms are difficult.

Note that although the movement of excess oxygen or an oxygen vacancyacross the layer of indium atoms is described above, the same can applyto the movement of excess oxygen or an oxygen vacancy across metalsother than indium which are contained in an oxide semiconductor film.

As described above, it is difficult for both excess oxygen and an oxygenvacancy to move across the layer of indium atoms, in other words, tomove in the c-axis direction. However, in a semiconductor device of oneembodiment of the present invention, forming oxygen vacancies in a stepof forming an oxide semiconductor film and using the oxygen vacancies asa diffusion path of excess oxygen enable a favorable supply of excessoxygen to the oxide semiconductor.

<1-5. Manufacturing Method of Semiconductor Device>

Next, a manufacturing method that is different from the manufacturingmethod described in <1-2. Manufacturing method of semiconductor device>will be described below with reference to FIGS. 10A and 10B. Note thatFIGS. 10A and 10B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device.

(6. Step of Removing the Protective Film)

First, steps illustrated in FIGS. 1B to 1E are performed. Then, theprotective film 130 is removed using an etchant 144, so that a surfaceof the oxide insulating film 114 is exposed (see FIG. 10A).

As the etchant 144, a chemical solution or an etching gas that canremove the protective film 130 is used. In this embodiment, a chemicalsolution, a 0.5% hydrofluoric acid, is used to remove the protectivefilm 130. Alternatively, a 5% oxalic acid solution may be used. Furtheralternatively, the protective film 130 may be etched by performingetching with a 5% oxalic acid solution and then a 0.5% hydrofluoricacid.

(7. Step of Forming a Nitride Insulating Film)

Next, a nitride insulating film 118 is formed over the oxide insulatingfilm 114 (see FIG. 10B).

The nitride insulating film 118 includes nitrogen. Alternatively, thenitride insulating film 118 includes nitrogen and silicon. The nitrideinsulating film 118 has a function of blocking oxygen, hydrogen, water,an alkali metal, an alkaline earth metal, or the like. With the nitrideinsulating film 118, it is possible to prevent outward diffusion ofoxygen from the oxide semiconductor film 108, outward diffusion ofoxygen included in the oxide insulating film 114, and entry of hydrogen,water, or the like into the oxide semiconductor film 108 and the oxideinsulating film 114 from the outside. The nitride insulating film 118may be silicon nitride, silicon nitride oxide, aluminum nitride,aluminum nitride oxide, or the like.

In the case of being formed by a PECVD method, the nitride insulatingfilm 118 is preferably formed at a temperature at which oxygen isreleased from the oxide insulating film 114, typically under conditionswhere the substrate temperature is higher than or equal to 150° C. andlower than or equal to 400° C.

For example, in the case where a silicon nitride film is formed by aPECVD method as the nitride insulating film 118, a deposition gascontaining silicon, nitrogen, and ammonia are preferably used as asource gas. When a small amount of ammonia compared to the amount ofnitrogen is used as the source gas, ammonia is dissociated in the plasmaand activated species are generated. The activated species cleave a bondbetween silicon and hydrogen which are contained in the deposition gascontaining silicon and a triple bond between nitrogen molecules. As aresult, a dense silicon nitride film having few defects, in which bondsbetween silicon and nitrogen are promoted and bonds between silicon andhydrogen are few, can be formed. On the other hand, when the amount ofammonia with respect to the amount of nitrogen is large, decompositionof the deposition gas containing silicon and decomposition of nitrogenare not promoted, so that a sparse silicon nitride film in which bondsbetween silicon and hydrogen remain and defects are increased is formed.Therefore, in the source gas, a flow rate ratio of the nitrogen to theammonia is set to be greater than or equal to 5 and less than or equalto 50, preferably greater than or equal to 10 and less than or equal to50.

In this embodiment, as the nitride insulating film 118, a 50-nm-thicksilicon nitride film is formed with a PECVD apparatus using silane,nitrogen, and ammonia as a source gas. The flow rate of silane is 50sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate ofammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa,the substrate temperature is 350° C., and a high-frequency power of 1000W is supplied to parallel-plate electrodes with a 27.12 MHzhigh-frequency power source. Note that the PECVD apparatus is aparallel-plate PECVD apparatus in which the electrode area is 6000 cm²,and the supplied power corresponds to a power per unit area (powerdensity) of 1.7×10⁻¹ W/cm².

The insulating film 118 may be deposited by heating, so that excessoxygen contained in the oxide insulating film 114 can be diffused intothe oxide semiconductor film 108 to fill oxygen vacancies in the oxidesemiconductor film 108.

In this embodiment, one embodiment of the present invention has beendescribed. However, one embodiment of the present invention is notlimited to the above-described examples. Although the example in whichan oxide semiconductor is used is described above for example as oneembodiment of the present invention, one embodiment of the presentinvention is not limited thereto. Depending on circumstances orconditions, one embodiment of the present invention is not necessarilyapplied to the case in which a different kind of semiconductor is used.Depending on circumstances or conditions, one embodiment of the presentinvention is not necessarily applied to the case in which an oxidesemiconductor is used. For example, although an example in which oxygenvacancies are filled to decrease the number of oxygen vacancies has beendescribed as one embodiment of the present invention, one embodiment ofthe present invention is not limited to this example. Depending oncircumstances or conditions, oxygen vacancies are not necessarilydecreased in one embodiment of the present invention. For example, anexample in which excess oxygen is diffused from a film provided over theoxide semiconductor film into the oxide semiconductor film has beendescribed as one embodiment of the present invention, one embodiment ofthe present invention is not limited to this example.

The structure and methods described in this embodiment can be combinedas appropriate with any of the other structures and methods described inthe other embodiments.

Embodiment 2

In this embodiment, semiconductor devices having structures differentfrom the semiconductor device described in Embodiment 1, and amanufacturing method thereof will be described with reference to FIGS.11A to 11C, FIGS. 12A to 12C, FIGS. 13A to 13C, FIGS. 14A to 14C, FIGS.15A to 15D, FIGS. 16A and 16B, FIGS. 17A to 17D, FIGS. 18A to 18D, FIGS.19A to 19D, FIGS. 20A and 20B, FIGS. 21A to 21D, and FIGS. 22A to 22D.

<2-1. Structural Example of Semiconductor Device>

FIG. 11A is a plan view of a transistor 200 that is a semiconductordevice of one embodiment of the present invention. FIG. 11B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 11A,and FIG. 11C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 11A. Note that in FIG. 11A, some components of thetransistor 200 (e.g., an insulating film serving as a gate insulatingfilm) are not illustrated to avoid complexity. Furthermore, thedirection of the dashed dotted line X1-X1 may be referred to as achannel length direction, and the direction of the dashed dotted lineY1-Y2 may be referred to as a channel width direction. As in FIG. 11A,some components are not illustrated in some cases in plan views oftransistors described below.

The transistor 200 includes a conductive film 204 functioning as a gateelectrode over a substrate 202, an insulating film 206 over thesubstrate 202 and the conductive film 204, an insulating film 207 overthe insulating film 206, an oxide semiconductor film 208 over theinsulating film 207, a conductive film 212 a functioning as a sourceelectrode electrically connected to the oxide semiconductor film 208,and a conductive film 212 b functioning as a drain electrodeelectrically connected to the oxide semiconductor film 208. Over thetransistor 200, specifically, over the conductive films 212 a and 212 band the oxide semiconductor film 208, an oxide insulating film 214 and anitride insulating film 218 are provided. The oxide insulating film 214and the nitride insulating film 218 function as protective insulatingfilms for the transistor 200.

Furthermore, the insulating films 206 and 207 function as gateinsulating films of the transistor 200. The oxide insulating film 214includes an oxide insulating film 214 a and an oxide insulating film 214b.

When oxygen vacancies are formed in the oxide semiconductor film 208included in the transistor 200, electrons serving as carriers aregenerated; as a result, the transistor 200 tends to be normally-on.Therefore, for stable transistor characteristics, it is important toreduce oxygen vacancies in the oxide semiconductor film 208. In the stepof forming the oxide semiconductor film 208 in the structure of thetransistor of one embodiment of the present invention, oxygen vacanciesare formed in the oxide semiconductor film 208. Then, oxygen is suppliedfrom the oxide insulating film 214 positioned over the oxidesemiconductor film 208, so that the oxygen vacancies are filled. Notethat in the step of forming the oxide semiconductor film 208, minutedefects or lattice defects may be formed at the same time as the oxygenvacancies in the oxide semiconductor film 208. Oxygen can be favorablysupplied from the oxide insulating film 214 to the oxide semiconductorfilm 208 by using the oxygen vacancies, the minute defects, or thelattice defects as a diffusion path of excess oxygen.

The substrate 202 may have a structure similar to that of the substrate102 described in Embodiment 1. The nitride insulating film 218 may havea structure similar to that of the nitride insulating film 118 describedin Embodiment 1. Other components of the transistor 200 will bedescribed below.

(Conductive Film)

The conductive film 204 functioning as a gate electrode and theconductive films 212 a and 212 b functioning as a source electrode and adrain electrode can each be formed using a metal element selected fromchromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc(Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W),manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloyincluding any of these metal elements as its component; an alloyincluding a combination of any of these metal elements; or the like.

The conductive films 204, 212 a, and 212 b may have a single-layerstructure or a stacked-layer structure of two or more layers. Forexample, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a titanium nitride film, a two-layerstructure in which a tungsten film is stacked over a tantalum nitridefilm or a tungsten nitride film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, an alloy film or anitride film in which aluminum and one or more elements selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium are combined may be used.

The conductive films 204, 212 a, and 212 b can also be formed using alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added.

A Cu-X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be usedfor the conductive films 204, 212 a, and 212 b. Use of a Cu-X alloy filmenables the manufacturing cost to be reduced because wet etching processcan be used in the processing.

(Gate Insulating Film)

As each of the insulating films 206 and 207 functioning as gateinsulating films of the transistor 200, an insulating layer including atleast one of the following films formed by a plasma enhanced chemicalvapor deposition (PECVD) method, a sputtering method, or the like can beused: a silicon oxide film, a silicon oxynitride film, a silicon nitrideoxide film, a silicon nitride film, an aluminum oxide film, a hafniumoxide film, an yttrium oxide film, a zirconium oxide film, a galliumoxide film, a tantalum oxide film, a magnesium oxide film, a lanthanumoxide film, a cerium oxide film, and a neodymium oxide film. Note thatinstead of the stacked-layer structure of the insulating films 206 and207, an insulating film of a single layer formed using a materialselected from the above or an insulating film of three or more layersmay be used.

Note that the insulating film 207 that is in contact with the oxidesemiconductor film 208 functioning as a channel region of the transistor200 is preferably an oxide insulating film and preferably includes aregion including oxygen in excess of the stoichiometric composition(oxygen-excess region). In other words, the insulating film 207 is aninsulating film capable of releasing oxygen. In order to provide theoxygen-excess region in the insulating film 207, the insulating film 207is formed in an oxygen atmosphere, for example. Alternatively, theoxygen-excess region may be formed by introduction of oxygen into theinsulating film 207 after the deposition. As a method for introducingoxygen, an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, plasma treatment, or the like may beemployed.

In the case where hafnium oxide is used as the insulating film 207, thefollowing effect is attained. Hafnium oxide has a higher dielectricconstant than silicon oxide and silicon oxynitride. Therefore, by usinghafnium oxide, the thickness of the insulating film 207 can be madelarge as compared with the case where silicon oxide is used; thus,leakage current due to tunnel current can be low. That is, it ispossible to provide a transistor with a low off-state current. Moreover,hafnium oxide with a crystalline structure has higher dielectricconstant than hafnium oxide with an amorphous structure. Therefore, itis preferable to use hafnium oxide with a crystalline structure in orderto provide a transistor with a low off-state current. Examples of thecrystalline structure include a monoclinic crystal structure and a cubiccrystal structure. Note that one embodiment of the present invention isnot limited thereto.

In this embodiment, a silicon nitride film is formed as the insulatingfilm 206, and a silicon oxide film is formed as the insulating film 207.The silicon nitride film has a higher dielectric constant than a siliconoxide film and needs a larger thickness for capacitance equivalent tothat of the silicon oxide film. Thus, when the silicon nitride film isincluded as the gate insulating film of the transistor 200, thethickness of the insulating film can be physically increased. This makesit possible to reduce a decrease in withstand voltage of the transistor200 and furthermore to increase the withstand voltage, thereby reducingelectrostatic discharge damage to the transistor 200.

(Oxide Semiconductor Film)

The oxide semiconductor film 208 may have a structure similar to that ofthe oxide semiconductor film 108 described in Embodiment 1. In addition,the oxide semiconductor film 208 preferably has the following structure.

The energy gap of the oxide semiconductor film 208 is 2 eV or more,preferably 2.5 eV or more, further preferably 3 eV or more. The use ofan oxide semiconductor having a wide energy gap can reduce off-statecurrent of the transistor 200.

For the oxide semiconductor film 208, a material with an appropriatecomposition may be used depending on required semiconductorcharacteristics and electrical characteristics (e.g., field-effectmobility and threshold voltage) of a transistor. Further, in order toobtain required semiconductor characteristics of a transistor, it ispreferable that the carrier density, the impurity concentration, thedefect density, the atomic ratio of a metal element to oxygen, theinteratomic distance, the density, and the like of the oxidesemiconductor film 208 be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 208,an oxide semiconductor film in which the impurity concentration is lowand the density of defect states is low, in which case the transistorcan have more excellent electrical characteristics. Here, the state inwhich the impurity concentration is low and the density of defect statesis low (the number of oxygen vacancies is small) is referred to as“highly purified intrinsic” or “substantially highly purifiedintrinsic”. A highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor film has few carrier generation sources,and thus can have a low carrier density. Thus, a transistor in which achannel region is formed in the oxide semiconductor film rarely has anegative threshold voltage (is rarely normally on). A highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorfilm has a low density of defect states and accordingly has a lowdensity of trap states in some cases. Further, the highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorfilm has an extremely low off-state current; even when an element has achannel width W of 1×10⁶ μm and a channel length L of 10 μm, theoff-state current can be less than or equal to the measurement limit ofa semiconductor parameter analyzer, that is, less than or equal to1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and adrain electrode of from 1 V to 10 V.

Accordingly, the transistor in which the channel region is formed in thehighly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film can have a small change in electricalcharacteristics and high reliability. Charges trapped by the trap statesin the oxide semiconductor film take a long time to be released and maybehave like fixed charges. Thus, the transistor whose channel region isformed in the oxide semiconductor film having a high density of trapstates has unstable electrical characteristics in some cases. Asexamples of the impurities, hydrogen, nitrogen, alkali metal, alkalineearth metal, and the like are given.

Hydrogen included in the oxide semiconductor film 208 reacts with oxygenbonded to a metal atom to be water, and also causes oxygen vacancies ina lattice from which oxygen is released (or a portion from which oxygenis released). Due to entry of hydrogen into the oxygen vacancies,electrons serving as carriers are generated in some cases. Furthermore,in some cases, bonding of part of hydrogen to oxygen bonded to a metalatom causes generation of an electron serving as a carrier. Thus, atransistor including an oxide semiconductor film which contains hydrogenis likely to be normally on. Accordingly, it is preferable that hydrogenbe reduced as much as possible in the oxide semiconductor film 208.Specifically, the hydrogen concentration of the oxide semiconductor film208, which is measured by secondary ion mass spectrometry (SIMS), islower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equalto 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹atoms/cm³, still further preferably lower than or equal to 5×10¹⁸atoms/cm³, yet further preferably lower than or equal to 1×10¹⁸atoms/cm³, even further preferably lower than or equal to 5×10¹⁷atoms/cm³, or further preferably lower than or equal to 1×10¹⁶atoms/cm³.

In addition, the concentration of alkali metal or alkaline earth metalof the oxide semiconductor film 208, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generatecarriers when bonded to an oxide semiconductor, in which case theoff-state current of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal of the oxide semiconductor film 208.

Furthermore, when including nitrogen, the oxide semiconductor film 208easily becomes n-type by generation of electrons serving as carriers andan increase of carrier density. Thus, a transistor including an oxidesemiconductor film which contains nitrogen is likely to have normally-oncharacteristics. For this reason, nitrogen in the oxide semiconductorfilm is preferably reduced as much as possible; the concentration ofnitrogen which is measured by SIMS is preferably set to be, for example,lower than or equal to 5×10¹⁸ atoms/cm³.

(Oxide Insulating Film)

The oxide insulating film 214 may have a structure similar to that ofthe oxide insulating film 114 described in Embodiment 1. In addition,the oxide insulating film 214 preferably has the following structure.

The oxide insulating film 214 functions as a protective insulating filmfor the oxide semiconductor film 208.

The oxide insulating film 214 a is preferably formed of a material thatreleases little nitrogen oxide. Nitrogen oxide (NO_(x); x is greaterthan or equal to 0 and less than or equal to 2, preferably greater thanor equal to 1 and less than or equal to 2), typically NO₂ or NO, formslevels in the oxide insulating film 214 a, for example. The level ispositioned in the energy gap of the oxide semiconductor film 208.Therefore, when nitrogen oxide is diffused to the vicinity of theinterface between the oxide insulating film 214 a and the oxidesemiconductor film 208, an electron is in some cases trapped by thelevel on the oxide insulating film 214 a side. As a result, the trappedelectron remains in the vicinity of the interface between the oxideinsulating film 214 a and the oxide semiconductor film 208; thus, thethreshold voltage of the transistor 200 is shifted in the positivedirection.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Sincenitrogen oxide included in the oxide insulating film 214 a reacts withammonia included in the oxide insulating film 214 b in heat treatment,nitrogen oxide included in the oxide insulating film 214 a is reduced.Therefore, an electron is hardly trapped at the vicinity of theinterface between the oxide insulating film 214 a and the oxidesemiconductor film 208.

By using such an oxide insulating film, the oxide insulating film 214 acan reduce the shift in the threshold voltage of the transistor 200,which leads to a smaller change in the electrical characteristics of thetransistor 200.

Note that in an ESR spectrum at 100 K or lower of the oxide insulatingfilm 214 a, by heat treatment of a manufacturing process of thetransistor 200, typically heat treatment at a temperature higher than orequal to 300° C. and lower than the strain point of the substrate, afirst signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, a second signal that appears at ag-factor of greater than or equal to 2.001 and less than or equal to2.003, and a third signal that appears at a g-factor of greater than orequal to 1.964 and less than or equal to 1.966 are observed. The splitwidth of the first and second signals and the split width of the secondand third signals that are obtained by ESR measurement using an X-bandare each approximately 5 mT. The sum of the spin densities of the firstsignal that appears at a g-factor of greater than or equal to 2.037 andless than or equal to 2.039, the second signal that appears at ag-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is lower than 1×10¹⁸spins/cm³, typically higher than or equal to 1×10¹⁷ spins/cm³ and lowerthan 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 correspond to signals attributed to nitrogenoxide (NO_(x); x is greater than or equal to 0 and less than or equal to2, preferably greater than or equal to 1 and less than or equal to 2).Typical examples of nitrogen oxide include nitrogen monoxide andnitrogen dioxide. In other words, the lower the total spin density ofthe first signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is, the lower thecontent of nitrogen oxide in the oxide insulating film 214 a is.

The concentration of nitrogen of the oxide insulating film 214 ameasured by SIMS is lower than or equal to 6×10²⁰ atoms/cm³.

The oxide insulating film 214 a formed by a PECVD method at a substratetemperature higher than or equal to 220° C., higher than or equal to280° C., or higher than or equal to 350° C. using silane and dinitrogenmonoxide can be a dense and hard film.

(Nitride Insulating Film)

The nitride insulating film 218 may have a structure similar to that ofthe nitride insulating film 118 described in Embodiment 1.

<2-2. Structural Example of Semiconductor Device>

A structure example different from that of the transistor 200 in FIGS.11A to 11C is described with reference to FIGS. 12A to 12C. Note that inthe case where a portion has a function similar to that described above,the same hatch pattern is applied to the portion, and the portion is notespecially denoted by a reference numeral in some cases.

FIG. 12A is a plan view of a transistor 250 that is a semiconductordevice of one embodiment of the present invention. FIG. 12B is across-sectional view taken along dashed-dotted line X1-X2 illustrated inFIG. 12A, and FIG. 12C is a cross-sectional view taken alongdashed-dotted line Y1-Y2 illustrated in FIG. 12A.

The transistor 250 includes the conductive film 204 functioning as agate electrode over the substrate 202, the insulating film 206 over thesubstrate 202 and the conductive film 204, the insulating film 207 overthe insulating film 206, the oxide semiconductor film 208 over theinsulating film 207, the oxide insulating film 214 over the oxidesemiconductor film 208, the conductive film 212 a functioning as asource electrode electrically connected to the oxide semiconductor film208 through an opening 251 a provided in the oxide insulating film 214,and the conductive film 212 b functioning as a drain electrodeelectrically connected to the oxide semiconductor film 208 through anopenings 251 b provided in the oxide insulating film 214. Over thetransistor 250, specifically over the conductive films 212 a and 212 band the oxide insulating film 214, the nitride insulating film 218 isprovided. The oxide insulating film 214 functions as a protectiveinsulating film for the oxide semiconductor film 208. The nitrideinsulating film 218 functions as a protective insulating film for thetransistor 250. Note that the oxide insulating film 214 includes theoxide insulating film 214 a and the oxide insulating film 214 b.

Although the transistor 200 has a channel-etched structure, thetransistor 250 in FIGS. 12A to 12C has a channel-protective structure.Thus, either the channel-etched structure or the channel-protectivestructure can be applied to the semiconductor device of one embodimentof the present invention. The other structures are the same as those ofthe transistor 200 and a similar effect can be obtained.

<2-3. Structural Example of Semiconductor Device>

A structure example different from that of the transistor 250 in FIGS.12A to 12C is described with reference to FIGS. 13A to 13C. Note that inthe case where a portion has a function similar to that described above,the same hatch pattern is applied to the portion, and the portion is notespecially denoted by a reference numeral in some cases.

FIG. 13A is a plan view of a transistor 260 that is a semiconductordevice of one embodiment of the present invention. FIG. 13B is across-sectional view taken along dashed-dotted line X1-X2 illustrated inFIG. 13A, and FIG. 13C is a cross-sectional view taken alongdashed-dotted line Y1-Y2 illustrated in FIG. 13A.

The transistor 260 includes the conductive film 204 functioning as agate electrode over the substrate 202, the insulating film 206 over thesubstrate 202 and the conductive film 204, the insulating film 207 overthe insulating film 206, the oxide semiconductor film 208 over theinsulating film 207, the oxide insulating film 214 over the oxidesemiconductor film 208, the conductive film 212 a functioning as asource electrode electrically connected to the oxide semiconductor film208, and the conductive film 212 b functioning as a drain electrodeelectrically connected to the oxide semiconductor film 208. Over thetransistor 260, specifically over the conductive films 212 a and 212 band the oxide insulating film 214, the nitride insulating film 218 isprovided. The oxide insulating film 214 functions as a protectiveinsulating film for the oxide semiconductor film 208. The nitrideinsulating film 218 functions as a protective insulating film for thetransistor 260. Note that the oxide insulating film 214 includes theoxide insulating film 214 a and the oxide insulating film 214 b.

The transistor 260 is different from the transistor 250 illustrated inFIGS. 12A to 12C in the shape of the oxide insulating film 214.Specifically, the oxide insulating film 214 of the transistor 260 has anisland shape over a channel region of the oxide semiconductor film 208.The other components are the same as those of the transistor 250, and asimilar effect is obtained.

<2-4. Structural Example of Semiconductor Device>

A structure example different from that of the transistor 200 in FIGS.11A to 11C is described with reference to FIGS. 14A to 14C. Note that inthe case where a portion has a function similar to that described above,the same hatch pattern is applied to the portion, and the portion is notespecially denoted by a reference numeral in some cases.

FIG. 14A is a plan view of a transistor 270 that is a semiconductordevice of one embodiment of the present invention. FIG. 14B is across-sectional view taken along dashed-dotted line X1-X2 illustrated inFIG. 14A, and FIG. 14C is a cross-sectional view taken alongdashed-dotted line Y1-Y2 illustrated in FIG. 14A.

The transistor 270 includes the conductive film 204 functioning as afirst gate electrode over the substrate 202, the insulating film 206over the substrate 202 and the conductive film 204, the insulating film207 over the insulating film 206, the oxide semiconductor film 208 overthe insulating film 207, the oxide insulating film 214 over the oxidesemiconductor film 208, the conductive film 212 a functioning as asource electrode electrically connected to the oxide semiconductor film208, the conductive film 212 b functioning as a drain electrodeelectrically connected to the oxide semiconductor film 208, the nitrideinsulating film 218 over the conductive films 212 a and 212 b and theoxide insulating film 214, and conductive films 220 a and 220 b over thenitride insulating film 218. Note that the oxide insulating film 214includes the oxide insulating film 214 a and the oxide insulating film214 b.

In the transistor 270, the oxide insulating film 214 and the nitrideinsulating film 218 functions as a second gate insulating film of thetransistor 270. Furthermore, the conductive film 220 a in the transistor270 functions as, for example, a pixel electrode used for a displaydevice. The conductive film 220 a is connected to the conductive film212 b through an opening 252 c provided in the oxide insulating film 214and the nitride insulating film 218. The conductive film 220 b in thetransistor 270 functions as a second gate electrode (also referred to asa back gate electrode).

As illustrated in FIG. 14C, the conductive film 220 b is connected tothe conductive film 204 functioning as a first gate electrode throughopenings 252 a and 252 b provided in the insulating films 206 and 207,the oxide insulating film 214, and the nitride insulating film 218.Accordingly, the conductive film 220 b and the conductive film 204 aresupplied with the same potential.

Note that although the structure in which the openings 252 a and 252 bare provided so that the conductive film 220 b and the conductive film204 are connected to each other is described in this embodiment, oneembodiment of the present invention is not limited thereto. For example,a structure in which only one of the openings 252 a and 252 b isprovided so that the conductive film 220 b and the conductive film 204are connected to each other, or a structure in which the openings 252 aand 252 b are not provided and the conductive film 220 b and theconductive film 204 are not connected to each other may be employed.Note that in the case where the conductive film 220 b and the conductivefilm 204 are not connected to each other, it is possible to applydifferent potentials to the conductive film 220 b and the conductivefilm 204.

As illustrated in FIG. 14B, the oxide semiconductor film 208 ispositioned to face each of the conductive film 204 functioning as afirst gate electrode and the conductive film 220 b functioning as asecond gate electrode, and is sandwiched between the two conductivefilms functioning as gate electrodes. The lengths in the channel lengthdirection and the channel width direction of the conductive film 220 bfunctioning as a second gate electrode are longer than those in thechannel length direction and the channel width direction of the oxidesemiconductor film 208. The whole oxide semiconductor film 208 iscovered with the conductive film 220 b with the oxide insulating film214 and the nitride insulating film 218 positioned therebetween. Sincethe conductive film 220 b functioning as a second gate electrode isconnected to the conductive film 204 functioning as a first gateelectrode through the openings 252 a and 252 b provided in theinsulating films 206 and 207, the oxide insulating film 214, and thenitride insulating film 218, a side surface of the oxide semiconductorfilm 208 in the channel width direction faces the conductive film 220 bfunctioning as a second gate electrode with the oxide insulating film214 and the nitride insulating film 218 positioned therebetween.

In other words, in the channel width direction of the transistor 270,the conductive film 204 functioning as a first gate electrode and theconductive film 220 b functioning as a second gate electrode areconnected to each other through the openings provided in the insulatingfilms 206 and 207 functioning as gate insulating films and the oxideinsulating film 214 and the nitride insulating film 218 functioning assecond gate insulating films; and the conductive film 204 and theconductive film 220 b surround the oxide semiconductor film 208 with theinsulating films 206 and 207 functioning as gate insulating films andthe oxide insulating film 214 and the nitride insulating film 218functioning as second gate insulating films positioned therebetween.

Such a structure enables the oxide semiconductor film 208 included inthe transistor 270 to be electrically surrounded by electric fields ofthe conductive film 204 functioning as a first gate electrode and theconductive film 220 b functioning as a second gate electrode. A devicestructure of a transistor, like that of the transistor 270, in whichelectric fields of a first gate electrode and a second gate electrodeelectrically surround an oxide semiconductor film where a channel regionis formed can be referred to as a surrounded channel (s-channel)structure.

Since the transistor 270 has the s-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor film 208 by the conductive film 204 functioning as a firstgate electrode; therefore, the current drive capability of thetransistor 270 can be improved and high on-state current characteristicscan be obtained. Since the on-state current can be increased, it ispossible to reduce the size of the transistor 270. In addition, sincethe transistor 270 is surrounded by the conductive film 204 functioningas a first gate electrode and the conductive film 220 b functioning as asecond gate electrode, the mechanical strength of the transistor 270 canbe increased.

<2-5. Structural Example of Semiconductor Device>

Structure examples different from that of the transistor 200 in FIGS.11A to 11C are described with reference to FIGS. 15A to 15D. Note thatin the case where a portion has a function similar to that describedabove, the same hatch pattern is applied to the portion, and the portionis not especially denoted by a reference numeral in some cases.

FIGS. 15A and 15B illustrate a cross-sectional view illustrating avariation of the transistor 270 in FIGS. 14B and 14C. FIGS. 15C and 15Dillustrate a cross-sectional view illustrating another variation of thetransistor 270 in FIGS. 14B and 14C.

A transistor 270A in FIGS. 15A and 15B has the same structure as thetransistor 270 in FIGS. 14B and 14C except that the oxide semiconductorfilm 208 has a three-layer structure. Specifically, the oxidesemiconductor film 208 of the transistor 270A includes an oxidesemiconductor film 208 a, an oxide semiconductor film 208 b, and anoxide semiconductor film 208 c.

A transistor 270B in FIGS. 15C and 15D has the same structure as thetransistor 270 in FIGS. 14B and 14C except that the oxide semiconductorfilm 208 has a two-layer structure. Specifically, the oxidesemiconductor film 208 of the transistor 270B includes the oxidesemiconductor film 208 b and the oxide semiconductor film 208 c.

Here, a band structure including the oxide semiconductor films 208 a,208 b, and 208 c and insulating films in contact with the oxidesemiconductor films is described with reference to FIGS. 16A and 16B.

FIG. 16A shows an example of a band structure in the thickness directionof a stacked-layer structure including the insulating film 207, theoxide semiconductor films 208 a, 208 b, and 208 c, and the oxideinsulating film 214 a. FIG. 16B shows an example of a band structure inthe thickness direction of a stacked-layer structure including theinsulating film 207, the oxide semiconductor films 208 b and 208 c, andthe oxide insulating film 214 a. For easy understanding, energy level ofthe conduction band minimum (Ec) of each of the insulating film 207, theoxide semiconductor films 208 a, 208 b, and 208 c, and the oxideinsulating film 214 a is shown in the band structures.

In the band structure of FIG. 16A, a silicon oxide film is used as eachof the insulating film 207 and the oxide insulating film 214 a, an oxidesemiconductor film formed using a metal oxide target having an atomicratio of metal elements, In:Ga:Zn=1:3:2, is used as the oxidesemiconductor film 208 a, an oxide semiconductor film formed using ametal oxide target having an atomic ratio of metal elements,In:Ga:Zn=1:1:1, is used as the oxide semiconductor film 208 b, and anoxide semiconductor film formed using a metal oxide target having anatomic ratio of metal elements, In:Ga:Zn=1:3:2, is used as the oxidesemiconductor film 208 c.

In the band structure of FIG. 16B, a silicon oxide film is used as eachof the insulating film 207 and the oxide insulating film 214 a, an oxidesemiconductor film formed using a metal oxide target having an atomicratio of metal elements, In:Ga:Zn=1:1:1, is used as the oxidesemiconductor film 208 b, and an oxide semiconductor film formed using ametal oxide target having an atomic ratio of metal elements,In:Ga:Zn=1:3:2, is used as the oxide semiconductor film 208 c.

As illustrated in FIGS. 16A and 16B, the energy level of the conductionband minimum gradually changes between the oxide semiconductor film 208a and the oxide semiconductor film 208 b and between the oxidesemiconductor film 208 b and the oxide semiconductor film 208 c. Inother words, the energy level of the conduction band minimum iscontinuously changed or continuously connected. To obtain such a bandstructure, there exists no impurity, which forms a defect state such asa trap center or a recombination center, at the interface between theoxide semiconductor film 208 a and the oxide semiconductor film 208 b orat the interface between the oxide semiconductor film 208 b and theoxide semiconductor film 208 c.

To form a continuous junction between the oxide semiconductor film 208 aand the oxide semiconductor film 208 b and between the oxidesemiconductor film 208 b and the oxide semiconductor film 208 c, it isnecessary to form the films successively without exposure to the air byusing a multi-chamber deposition apparatus (sputtering apparatus)provided with a load lock chamber.

With the band structure of FIG. 16A or FIG. 16B, the oxide semiconductorfilm 208 b serves as a well, and a channel region is formed in the oxidesemiconductor film 208 b in the transistor with the stacked-layerstructure.

By providing the oxide semiconductor film 208 a and the oxidesemiconductor film 208 c, the oxide semiconductor film 208 b can bedistanced away from trap states.

In addition, the trap states might be more distant from the vacuum levelthan the energy level of the conduction band minimum (Ec) of the oxidesemiconductor film 208 b functioning as a channel region, so thatelectrons are likely to be accumulated in the trap states. When theelectrons are accumulated in the trap states, the electrons becomenegative fixed electric charge, so that the threshold voltage of thetransistor is shifted in the positive direction. Therefore, it ispreferable that the trap states be closer to the vacuum level than theenergy level of the conduction band minimum (Ec) of the oxidesemiconductor film 208 b. Such a structure inhibits accumulation ofelectrons in the trap states. As a result, the on-state current and thefield-effect mobility of the transistor can be increased.

The energy level of the conduction band minimum of each of the oxidesemiconductor films 208 a and 208 c is closer to the vacuum level thanthat of the oxide semiconductor film 208 b. Typically, a difference inenergy level between the conduction band minimum of the oxidesemiconductor film 208 b and the conduction band minimum of each of theoxide semiconductor films 208 a and 208 c is 0.15 eV or more or 0.5 eVor more and 2 eV or less or 1 eV or less. That is, the differencebetween the electron affinity of each of the oxide semiconductor films208 a and 208 c and the electron affinity of the oxide semiconductorfilm 208 b is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eVor less.

In such a structure, the oxide semiconductor film 208 b serves as a mainpath of current and functions as a channel region. In addition, sincethe oxide semiconductor films 208 a and 208 c each include one or moremetal elements included in the oxide semiconductor film 208 b in which achannel region is formed, interface scattering is less likely to occurat the interface between the oxide semiconductor film 208 a and theoxide semiconductor film 208 b or at the interface between the oxidesemiconductor film 208 b and the oxide semiconductor film 208 c. Thus,the transistor can have high field-effect mobility because the movementof carriers is not hindered at the interface.

To prevent each of the oxide semiconductor films 208 a and 208 c fromfunctioning as part of a channel region, a material having sufficientlylow conductivity is used for the oxide semiconductor films 208 a and 208c. Alternatively, a material which has a smaller electron affinity (adifference in energy level between the vacuum level and the conductionband minimum) than the oxide semiconductor film 208 b and has adifference in energy level in the conduction band minimum from the oxidesemiconductor film 208 b (band offset) is used for the oxidesemiconductor films 208 a and 208 c. Furthermore, to inhibit generationof a difference in threshold voltage due to the value of the drainvoltage, it is preferable to form the oxide semiconductor films 208 aand 208 c using a material whose energy level of the conduction bandminimum is closer to the vacuum level than that of the oxidesemiconductor film 208 b by 0.2 eV or more, preferably 0.5 eV or more.

It is preferable that the oxide semiconductor films 208 a and 208 c nothave a spinel crystal structure. This is because if the oxidesemiconductor films 208 a and 208 c have a spinel crystal structure,constituent elements of the conductive films 212 a and 212 b might bediffused to the oxide semiconductor film 208 b at the interface betweenthe spinel crystal structure and another region. Note that each of theoxide semiconductor film 208 a and 208 c is preferably a CAAC-OS, whichis described later, in which case a higher blocking property againstconstituent elements of the conductive films 212 a and 212 b, forexample, copper elements, is obtained.

The thickness of each of the oxide semiconductor films 208 a and 208 cis greater than or equal to a thickness that is capable of inhibitingdiffusion of the constituent elements of the conductive films 212 a and212 b to the oxide semiconductor film 208 b, and less than a thicknessthat inhibits supply of oxygen from the oxide insulating film 214 a tothe oxide semiconductor film 208 b. For example, when the thickness ofeach of the oxide semiconductor films 208 a and 208 c is greater than orequal to 10 nm, diffusion of the constituent elements of the conductivefilms 212 a and 212 b to the oxide semiconductor film 208 b can beinhibited. When the thickness of each of the oxide semiconductor films208 a and 208 c is less than or equal to 100 nm, oxygen can beeffectively supplied from the oxide insulating film 214 a to the oxidesemiconductor film 208 b.

When the oxide semiconductor films 208 a and 208 c are each an In-M-Znoxide in which the atomic ratio of the element M (M is Ti, Ga, Y, Zr,La, Ce, Nd, Sn, or Hf) is higher than that of In, the energy gap of eachof the oxide semiconductor films 208 a and 208 c can be large and theelectron affinity thereof can be small. Therefore, a difference inelectron affinity between the oxide semiconductor film 208 b and each ofthe oxide semiconductor films 208 a and 208 c may be controlled by theproportion of the element M. Furthermore, oxygen vacancies are lesslikely to be generated in the oxide semiconductor film in which theatomic ratio of Ti, Ga, Y, Zr, La, Ce, Nd, Sn or Hf is higher than thatof In because Ti, Ga, Y, Zr, La, Ce, Nd, Sn, and Hf are each a metalelement that is strongly bonded to oxygen.

When an In-M-Zn oxide is used for the oxide semiconductor films 208 aand 208 c, the proportions of In and M, not taking Zn and O intoconsideration, is as follows: the atomic percentage of In is preferablyless than 50 atomic % and the atomic percentage of M is greater than 50atomic % and further preferably the atomic percentage of In is less than25 atomic % and the atomic percentage of M is greater than 75 atomic %.Alternatively, a gallium oxide film may be used as each of the oxidesemiconductor films 208 a and 208 c.

Furthermore, in the case where each of the oxide semiconductor films 208a, 208 b, and 208 c is an In-M-Zn oxide, the proportion of M atoms ineach of the oxide semiconductor films 208 a and 208 c is higher thanthat in the oxide semiconductor film 208 b. Typically, the proportion ofM atoms in each of the oxide semiconductor films 208 a and 208 c is 1.5or more times, preferably two or more times and further preferably threeor more times as high as that in the oxide semiconductor film 208 b.

Furthermore, in the case where the oxide semiconductor films 208 a, 208b, and 208 c are each an In-M-Zn oxide, when the oxide semiconductorfilm 208 b has an atomic ratio of In:M:Zn=x₁:y₁:z₁ and the oxidesemiconductor films 208 a and 208 c each have an atomic ratio ofIn:M:Zn=x₂:y₂:z₂, y₂/x₂ is larger than y₁/x₁, preferably y₂/x₂ is 1.5 ormore times as large as y₁/x₁, further preferably, y₂/x₂ is two or moretimes as large as y₁/x₁, and still further preferably y₂/x₂ is three ormore times or four or more times as large as y₁/x₁. At this time, y₁ ispreferably greater than or equal to x₁ in the oxide semiconductor film208 b, because stable electrical characteristics of a transistorincluding the oxide semiconductor film 208 b can be achieved. However,when y₁ is three or more times as large as x₁, the field-effect mobilityof the transistor including the oxide semiconductor film 208 b isreduced. Accordingly, y₁ is preferably smaller than three times x₁.

In the case where the oxide semiconductor film 208 b is an In-M-Zn oxideand a target having the atomic ratio of metal elements ofIn:M:Zn=x₁:y₁:z₁ is used for depositing the oxide semiconductor film 208b, x₁/y₁ is preferably greater than or equal to 1/3 and less than orequal to 6 and further preferably greater than or equal to 1 and lessthan or equal to 6, and z₁/y₁ is preferably greater than or equal to 1/3and less than or equal to 6 and further preferably greater than or equalto 1 and less than or equal to 6. Note that when z₁/y₁ is greater thanor equal to 1 and less than or equal to 6, a CAAC-OS to be describedlater is easily formed as the oxide semiconductor film 208 b. Typicalexamples of the atomic ratio of the metal elements of the target areIn:M:Zn=1:1:1, In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2.

In the case where the oxide semiconductor films 208 a and 208 c are eachan In-M-Zn oxide and a target having an atomic ratio of metal elementsof In:M:Zn=x₂:y₂:z₂ is used for depositing the oxide semiconductor films208 a and 208 c, x₂/y₂ is preferably less than x₁/y₁, and z₂/y₂ ispreferably greater than or equal to 1/3 and less than or equal to 6 andfurther preferably greater than or equal to 1 and less than or equal to6. When the atomic ratio of M with respect to In is high, the energy gapof the oxide semiconductor films 208 a and 208 c can be large and theelectron affinity thereof can be small; therefore, y₂/x₂ is preferablyhigher than or equal to 3 or higher than or equal to 4. Typical examplesof the atomic ratio of the metal elements of the target includeIn:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:5, In:M:Zn=1:3:6,In:M:Zn=1:4:2, In:M:Zn=1:4:4, In:M:Zn=1:4:5, and In:M:Zn=1:5:5.

Furthermore, in the case where the oxide semiconductor films 208 a and208 c are each an In-M oxide, when a divalent metal element (e.g., zinc)is not included as M, the oxide semiconductor films 208 a and 208 cwhich do not include a spinel crystal structure can be formed. As theoxide semiconductor films 208 a and 208 c, for example, an In—Ga oxidefilm can be used. The In—Ga oxide can be deposited by a sputteringmethod using an In—Ga metal oxide target (In:Ga=7:93), for example. Todeposit the oxide semiconductor films 208 a and 208 c by a sputteringmethod using DC discharge, on the assumption that an atomic ratio ofIn:M is x:y, y/(x+y) is preferably less than or equal to 0.96 andfurther preferably less than or equal to 0.95, for example, 0.93.

In each of the oxide semiconductor films 208 a, 208 b, and 208 c, theproportions of the atoms in the above atomic ratio vary within an errorof approximately ±40%.

The structures of the transistors of this embodiment can be freelycombined with each other.

<2-6. Manufacturing Method of Semiconductor Device>

Next, a method for manufacturing the transistor 200 that is asemiconductor device of one embodiment of the present invention will bedescribed in detail below with reference to FIGS. 17A to 17D and FIGS.18A to 18D.

First, a conductive film is formed over the substrate 202 and processedthrough a lithography process and an etching process, whereby theconductive film 204 functioning as a gate electrode is formed. Then, theinsulating films 206 and 207 functioning as gate insulating films areformed over the conductive film 204 (see FIG. 17A).

The conductive film 204 functioning as a gate electrode can be formed bya sputtering method, a chemical vapor deposition (CVD) method, a vacuumevaporation method, or a pulsed laser deposition (PLD) method.Alternatively, a coating method or a printing method can be used.Although typical deposition methods are a sputtering method and a plasmaenhanced CVD (PECVD) method, a thermal CVD method, such as a metalorganic CVD (MOCVD) method, or an atomic layer deposition (ALD) methoddescribed above may be used.

In this embodiment, a glass substrate is used as the substrate 202, andas the conductive film 204 functioning as a gate electrode, a100-nm-thick tungsten film is formed by a sputtering method.

The insulating films 206 and 207 functioning as gate insulating filmscan be formed by a sputtering method, a PECVD method, a thermal CVDmethod, a vacuum evaporation method, a PLD method, or the like. In thisembodiment, a 400-nm-thick silicon nitride film as the insulating film206 and a 50-nm-thick silicon oxynitride film as the insulating film 207are formed by a PECVD method.

Note that the insulating film 206 can have a stacked-layer structure ofsilicon nitride films. Specifically, the insulating film 206 can have athree-layer structure of a first silicon nitride film, a second siliconnitride film, and a third silicon nitride film. An example of thethree-layer structure is as follows.

For example, the first silicon nitride film can be formed to have athickness of 50 nm under the conditions where silane at a flow rate of200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at aflow rate of 100 sccm are supplied as a source gas to a reaction chamberof a PECVD apparatus, the pressure in the reaction chamber is controlledto 100 Pa, and the power of 2000 W is supplied using a 27.12 MHzhigh-frequency power source.

The second silicon nitride film can be formed to have a thickness of 300nm under the conditions where silane at a flow rate of 200 sccm,nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rateof 2000 sccm are supplied as a source gas to the reaction chamber of thePECVD apparatus, the pressure in the reaction chamber is controlled to100 Pa, and the power of 2000 W is supplied using a 27.12 MHzhigh-frequency power source.

The third silicon nitride film can be formed to have a thickness of 50nm under the conditions where silane at a flow rate of 200 sccm andnitrogen at a flow rate of 5000 sccm are supplied as a source gas to thereaction chamber of the PECVD apparatus, the pressure in the reactionchamber is controlled to 100 Pa, and the power of 2000 W is suppliedusing a 27.12 MHz high-frequency power source.

Note that the first silicon nitride film, the second silicon nitridefilm, and the third silicon nitride film can each be formed at asubstrate temperature of 350° C.

When the insulating film 206 has the three-layer structure of siliconnitride films, for example, in the case where a conductive filmincluding Cu is used as the conductive film 204, the following effectcan be obtained.

The first silicon nitride film can inhibit diffusion of a copper (Cu)element from the conductive film 204. The second silicon nitride filmhas a function of releasing hydrogen and can improve withstand voltageof the insulating film functioning as a gate insulating film. The thirdsilicon nitride film releases a small amount of hydrogen and can inhibitdiffusion of hydrogen released from the second silicon nitride film.

The insulating film 207 is preferably an insulating film includingoxygen to improve characteristics of an interface with the oxidesemiconductor film 208 formed later.

Next, the oxide semiconductor film 208 is formed over the insulatingfilm 207 (see FIG. 17B).

In this embodiment, an oxide semiconductor film is deposited with asputtering apparatus using an In—Ga—Zn metal oxide target (with anatomic ratio of In:Ga:Zn=4:2:4.1). The oxide semiconductor film isformed under conditions where oxygen vacancies are formed. When theoxide semiconductor film is formed under the conditions where thepartial pressure of oxygen in the sputtering apparatus is higher than 0%and lower than 50%, oxygen vacancies in the oxide semiconductor film canbe increased. In this embodiment, the oxide semiconductor film is formedunder at an oxygen partial pressure in the sputtering apparatus of 30%.Next, a mask is formed over the oxide semiconductor film through alithography process, and the oxide semiconductor film is processed intoa desired shape to form the island-shaped oxide semiconductor film 208.

After the island-shaped oxide semiconductor film 208 is formed, aheating step may be performed to increase oxygen vacancies in the oxidesemiconductor film 208. The heating step can be performed in a mannersimilar to that of the heating step after formation of the oxidesemiconductor film 108 in Embodiment 1.

Next, the conductive films 212 a and 212 b which function as source anddrain electrodes are formed over the insulating film 207 and the oxidesemiconductor film 208 (see FIG. 17C).

In this embodiment, the conductive films 212 a and 212 b are formed inthe following manner: a stack formed of a 50-nm-thick tungsten film anda 400-nm-thick aluminum film is formed by a sputtering method, a mask isformed over the stack through a lithography process, and the stack isprocessed into desired regions. Although the conductive films 212 a and212 b each have a two-layer structure in this embodiment, one embodimentof the present invention is not limited thereto. For example, theconductive films 212 a and 212 b each may have a three-layer structureof a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a100-nm-thick titanium film.

After the conductive films 212 a and 212 b are formed, a surface of theoxide semiconductor film 208 (on the back channel side) may be cleaned.The cleaning may be performed, for example, using a chemical solutionsuch as phosphoric acid. The cleaning using a chemical solution such asa phosphoric acid can remove impurities (e.g., an element included inthe conductive films 212 a and 212 b) attached to the surface of theoxide semiconductor film 208.

As illustrated in FIGS. 11B and 11C and FIG. 17C, in the step of formingthe conductive films 212 a and 212 b and/or the cleaning step, arecessed portion might be formed in part of the oxide semiconductor film208.

Note that the cross-sectional structure when a recessed portion is notformed in part of the oxide semiconductor film 208 at the step offorming the conductive films 212 a and 212 b and/or the cleaning step isillustrated in FIGS. 45A and 45B.

Through the above-described steps, the transistor 200 is formed.

Next, over the transistor 200, specifically, over the oxidesemiconductor film 208 and the conductive films 212 a and 212 b of thetransistor 200, the oxide insulating film 214 (the oxide insulating film214 a and the oxide insulating film 214 b) functioning as a protectiveinsulating film of the transistor 200 is formed (see FIG. 17D).

The oxide insulating films 214 a and 214 b can be formed using amaterial and a method similar to those of the oxide insulating films 114a and 114 b described in Embodiment 1.

After the oxide insulating film 214 is formed, heat treatment may beperformed. The heat treatment can reduce nitrogen oxide included in theoxide insulating film 214. By the heat treatment, part of oxygenincluded in the oxide insulating film 214 can be moved to the oxidesemiconductor film 208, so that the amount of oxygen vacancies includedin the oxide semiconductor film 208 can be reduced. In this embodiment,the heat treatment is performed at 350° C. for 1 hour in an atmosphereof nitrogen and oxygen.

Next, a protective film 230 is formed over the oxide insulating film 214(see FIG. 18A).

The protective film 230 can be formed using a material and a methodsimilar to those of the protective film 130 described in Embodiment 1.

Next, oxygen 242 is added to the oxide insulating film 214 through theprotective film 230 (see FIG. 18B).

As the method for adding the oxygen 242, a method similar to the methodfor adding the oxygen 142 described in Embodiment 1 may be used.

Then, the protective film 230 is removed using an etchant 244 (see FIG.18C).

As the method for removing the protective film 230, a method similar tothe method for removing the protective film 130 in Embodiment 1 may beused.

Next, the nitride insulating film 218 is formed over the oxideinsulating film 214. In this way, the transistor 200 illustrated inFIGS. 11A to 11C can be formed (see FIG. 18D).

The nitride insulating film 218 can be formed using a material and amethod similar to those of the nitride insulating film 118 described inEmbodiment 1.

Through the above-described steps, the transistor 200 illustrated inFIGS. 11A to 11C can be formed.

<2-7. Manufacturing Method of Semiconductor Device>

Next, a method for manufacturing the transistor 250 illustrated in FIGS.12A to 12C will be described in detail below with reference to FIGS. 19Ato 19D and FIGS. 20A and 20B.

First, the steps up to the step illustrated in FIG. 17B are performed,and then the oxide insulating film 214 and the protective film 230 areformed over the insulating film 207 and the oxide semiconductor film 208(see FIG. 19A).

Then, the oxygen 242 is added to the oxide insulating film 214 throughthe protective film 230 (see FIG. 19B).

Then, the protective film 230 is removed using the etchant 244 (see FIG.19C)

Next, a mask is formed over the oxide insulating film 214 through alithography process, and the openings 251 a and 251 b are formed indesired regions in the oxide insulating film 214. Note that the openings251 a and 251 b reach the oxide semiconductor film 208 (see FIG. 19D).

Next, a conductive film is deposited over the oxide semiconductor film208 and the oxide insulating film 214 to cover the openings 251 a and251 b, a mask is formed over the conductive film through a lithographyprocess, and the conductive film is processed into desired regions,whereby the conductive films 212 a and 212 b are formed (see FIG. 20A).

Next, the nitride insulating film 218 is formed over the oxideinsulating film 214 and the conductive films 212 a and 212 b (see FIG.20B).

Through the above-described steps, the transistor 250 illustrated inFIGS. 12A to 12C can be manufactured.

Note that the transistor 260 in FIGS. 13A to 13C can be manufactured insuch a manner that the oxide insulating film 214 is left over a channelregion of the oxide semiconductor film 208 at the step of forming theopenings 251 a and 251 b.

<2-8. Manufacturing Method of Semiconductor Device>

Next, a method for manufacturing the transistor 270 illustrated in FIGS.14A to 14C will be described in detail below with reference to FIGS. 21Ato 21D and FIGS. 22A to 22D.

FIGS. 21A and 21C and FIGS. 22A and 22C are each a cross-sectional viewin the channel length direction of the transistor 270 in themanufacturing process, and FIGS. 21B and 21D and FIGS. 22B and 22D areeach a cross-sectional view in the channel width direction of thetransistor 270 in the manufacturing process.

First, the steps up to the step illustrated in FIG. 18D are performed(see FIGS. 21A and 21B).

Next, a mask is formed over the nitride insulating film 218 through alithography process, and the opening 252 c is formed in a desired regionin the oxide insulating film 214 and the nitride insulating film 218. Amask is formed over the nitride insulating film 218 through alithography process, and the openings 252 a and 252 b are formed indesired regions in the insulating films 206 and 207, the oxideinsulating film 214, and the nitride insulating film 218. Note that theopening 252 c reaches the conductive film 212 b. The openings 252 a and252 b reach the conductive film 204 (see FIGS. 21C and 21D).

Note that the openings 252 a and 252 b and the opening 252 c may beformed in the same step or may be formed by different steps. In the casewhere the openings 252 a and 252 b and the opening 252 c are formed inthe same step, for example, a gray-tone mask or a half-tone mask can beused.

Then, a conductive film 220 is formed over the nitride insulating film218 so as to cover the openings 252 a, 252 b, and 252 c (see FIGS. 22Aand 22B).

For the conductive film 220, for example, a material including one ofindium (In), zinc (Zn), and tin (Sn) can be used. In particular, for theconductive film 220, a light-transmitting conductive material such asindium oxide including tungsten oxide, indium zinc oxide includingtungsten oxide, indium oxide including titanium oxide, indium tin oxideincluding titanium oxide, indium tin oxide, indium zinc oxide, or indiumtin oxide including silicon oxide can be used. It is preferable to formthe conductive film 220 and the protective film 230 using the samematerial because manufacturing cost can be suppressed.

The conductive film 220 can be formed with a sputtering apparatus, forexample. In this embodiment, a 110-nm-thick ITSO film is formed with asputtering apparatus.

Next, a mask is formed over the conductive film 220 through alithography process, and the conductive film 220 is processed intodesired shapes to form the conductive films 220 a and 220 b (see FIGS.22C and 22D).

Through the above-described steps, the transistor 270 illustrated inFIGS. 14A to 14C can be manufactured.

Although films included in a transistor (a variety of films such as aconductive film, an insulating film, and an oxide semiconductor film)can be formed by a sputtering method or a PECVD method, such films maybe formed by another method, e.g., a thermal chemical vapor deposition(CVD) method. Examples of the thermal CVD method include a metal organicchemical vapor deposition (MOCVD) method and an atomic layer deposition(ALD) method. Alternatively, a variety of films such as a conductivefilm, an insulating film, and an oxide semiconductor film may be formedby a coating method or a printing method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a timeso that the pressure in a chamber is set to an atmospheric pressure or areduced pressure, and react with each other in the vicinity of thesubstrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a first layer;then the second source gas is introduced to react with the first layer;as a result, a second layer is stacked over the first layer, so that athin film is formed. The sequence of the gas introduction is repeated aplurality of times until a desired thickness is obtained, whereby a thinfilm with excellent step coverage can be formed. The thickness of thethin film can be adjusted by the number of repetition times of thesequence of the gas introduction; therefore, an ALD method makes itpossible to accurately adjust a thickness and thus is suitable formanufacturing a minute FET. By a thermal CVD method such as an MOCVDmethod or an ALD method, a variety of films such as the conductive film,the insulating film, the oxide semiconductor film, and the metal oxidefilm in the above embodiment can be formed.

The structures and methods described in this embodiment can be combinedas appropriate with any of the other structures and methods described inthe other embodiments.

Embodiment 3

In this embodiment, the structure of an oxide semiconductor filmincluded in a semiconductor device of one embodiment of the presentinvention is described in detail. First, structures that can be includedin an oxide semiconductor film are described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor.

Examples of a non-single-crystal oxide semiconductor include a CAAC-OS(c-axis aligned crystalline oxide semiconductor), a polycrystallineoxide semiconductor, a microcrystalline oxide semiconductor, and anamorphous oxide semiconductor. In addition, examples of a crystallineoxide semiconductor include a single crystal oxide semiconductor, aCAAC-OS, a polycrystalline oxide semiconductor, and a microcrystallineoxide semiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and a microcrystalline oxide semiconductor.

<3-1. CAAC-OS>

First, a CAAC-OS is described. Note that a CAAC-OS can be referred to asan oxide semiconductor including CANC (c-axis aligned nanocrystals).

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

FIG. 24A shows an example of a high-resolution TEM image of a crosssection of the CAAC-OS which is obtained from a direction substantiallyparallel to the sample surface. Here, the TEM image is obtained with aspherical aberration corrector function. The high-resolution TEM imageobtained with a spherical aberration corrector function is particularlyreferred to as a Cs-corrected high-resolution TEM image in the followingdescription. Note that the Cs-corrected high-resolution TEM image can beobtained with, for example, an atomic resolution analytical electronmicroscope JEM-ARM200F manufactured by JEOL Ltd.

A CAAC-OS observed with TEM is described below. FIG. 24A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 24B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 24A. FIG. 24B shows that metal atoms are arranged ina layered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which the CAAC-OS is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 24B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 24C. FIGS. 24B and 24C prove that the size of apellet is approximately 1 nm to 3 nm, and the size of a space caused bytilt of the pellets is approximately 0.8 nm. Therefore, the pellet canalso be referred to as a nanocrystal (nc).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 24D). The part in which the pellets are tilted as observed inFIG. 24C corresponds to a region 5161 shown in FIG. 24D.

FIG. 25A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 25B, 25C, and 25D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 25A,respectively. FIGS. 25B, 25C, and 25D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalis analyzed by an out-of-plane method, a peak appears at a diffractionangle (2θ) of around 31° as shown in FIG. 26A. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak at 2θ of around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is attributed to the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (ϕ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (ϕ axis), as shown in FIG. 26B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when ϕ scan is performed with2θ fixed at around 56°, as shown in FIG. 26C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 27A can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 27B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 27B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 27B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.The second ring in FIG. 27B is considered to be derived from the (110)plane and the like.

Moreover, the CAAC-OS is an oxide semiconductor having a low density ofdefect states. Defects in the oxide semiconductor are, for example, adefect due to impurity and oxygen vacancies. Therefore, the CAAC-OS canbe regarded as an oxide semiconductor with a low impurity concentration,or an oxide semiconductor having a small number of oxygen vacancies.

The impurity contained in the oxide semiconductor might serve as acarrier trap or serve as a carrier generation source. Furthermore,oxygen vacancies in the oxide semiconductor serve as carrier traps orserve as carrier generation sources when hydrogen is captured therein.

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

An oxide semiconductor having a low density of defect states (a smallnumber of oxygen vacancies) can have a low carrier density. Such anoxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.That is, a CAAC-OS is likely to be a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. Thus, atransistor including a CAAC-OS rarely has negative threshold voltage (israrely normally on). The highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor has few carrier traps. Anelectric charge trapped by the carrier traps in the oxide semiconductortakes a long time to be released. The trapped electric charge may behavelike a fixed electric charge. Thus, the transistor which includes theoxide semiconductor having a high impurity concentration and a highdensity of defect states might have unstable electrical characteristics.However, a transistor including a CAAC-OS has small variation inelectrical characteristics and high reliability.

Since the CAAC-OS has a low density of defect states, carriers are lesslikely to be trapped in defect states with light irradiation. Therefore,in a transistor using the CAAC-OS, change in electrical characteristicsdue to irradiation with visible light or ultraviolet light is small.

<3-2. Microcrystalline Oxide Semiconductor>

Next, a microcrystalline oxide semiconductor is described.

A microcrystalline oxide semiconductor has a region in which a crystalpart is observed and a region in which a crystal part is not clearlyobserved in a high-resolution TEM image. In most cases, the size of acrystal part included in the microcrystalline oxide semiconductor isgreater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. An oxidesemiconductor including a nanocrystal (nc) that is a microcrystal with asize greater than or equal to 1 nm and less than or equal to 10 nm, or asize greater than or equal to 1 nm and less than or equal to 3 nm isspecifically referred to as a nanocrystalline oxide semiconductor(nc-OS). In a high-resolution TEM image of the nc-OS, for example, agrain boundary is not clearly observed in some cases. Note that there isa possibility that the origin of the nanocrystal is the same as that ofa pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may bereferred to as a pellet in the following description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS cannot be distinguished from an amorphous oxidesemiconductor, depending on an analysis method. For example, when thenc-OS is subjected to structural analysis by an out-of-plane method withan XRD apparatus using an X-ray having a diameter larger than the sizeof a pellet, a peak which shows a crystal plane does not appear.Furthermore, a diffraction pattern like a halo pattern is observed whenthe nc-OS is subjected to electron diffraction using an electron beamwith a probe diameter (e.g., 50 nm or larger) that is larger than thesize of a pellet (the electron diffraction is also referred to asselected-area electron diffraction). Meanwhile, spots appear in ananobeam electron diffraction pattern of the nc-OS when an electron beamhaving a probe diameter close to or smaller than the size of a pellet isapplied. Moreover, in a nanobeam electron diffraction pattern of thenc-OS, regions with high luminance in a circular (ring) pattern areshown in some cases. Also in a nanobeam electron diffraction pattern ofthe nc-OS, a plurality of spots is shown in a ring-like region in somecases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an amorphous oxidesemiconductor. Note that there is no regularity of crystal orientationbetween different pellets in the nc-OS. Therefore, the nc-OS has ahigher density of defect states than the CAAC-OS.

<3-3. Amorphous Oxide Semiconductor>

Next, an amorphous oxide semiconductor is described.

The amorphous oxide semiconductor is an oxide semiconductor havingdisordered atomic arrangement and no crystal part and exemplified by anoxide semiconductor which exists in an amorphous state as quartz.

In a high-resolution TEM image of the amorphous oxide semiconductor,crystal parts cannot be found.

When the amorphous oxide semiconductor is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is observed whenthe amorphous oxide semiconductor is subjected to electron diffraction.Furthermore, a spot is not observed and only a halo pattern appears whenthe amorphous oxide semiconductor is subjected to nanobeam electrondiffraction.

There are various understandings of an amorphous structure. For example,a structure whose atomic arrangement does not have ordering at all iscalled a completely amorphous structure. Meanwhile, a structure whichhas ordering within the nearest neighbor atomic distance or thesecond-nearest neighbor atomic distance but does not have long-rangeordering is also called an amorphous structure. Therefore, the strictestdefinition does not permit an oxide semiconductor to be called anamorphous oxide semiconductor as long as even a negligible degree ofordering is present in an atomic arrangement. At least an oxidesemiconductor having long-term ordering cannot be called an amorphousoxide semiconductor. Accordingly, because of the presence of crystalpart, for example, a CAAC-OS and an nc-OS cannot be called an amorphousoxide semiconductor or a completely amorphous oxide semiconductor.

<3-4. Amorphous-Like Oxide Semiconductor>

Note that an oxide semiconductor may have a structure having physicalproperties intermediate between the nc-OS and the amorphous oxidesemiconductor. The oxide semiconductor having such a structure isspecifically referred to as an amorphous-like oxide semiconductor(a-like OS).

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it contains a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (referred to as Sample A), an nc-OS (referred to as SampleB), and a CAAC-OS (referred to as Sample C) are prepared as samplessubjected to electron irradiation. Each of the samples is an In—Ga—Znoxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that a crystal part is determined as follows. It is known that aunit cell of an InGaZnO₄ crystal has a structure in which nine layersincluding three In—O layers and six Ga—Zn—O layers are stacked in thec-axis direction. The distance between the adjacent layers is equivalentto the lattice spacing on the (009) plane (also referred to as d value).The value is calculated to be 0.29 nm from crystal structural analysis.Accordingly, a portion where the lattice spacing between lattice fringesis greater than or equal to 0.28 nm and less than or equal to 0.30 nm isregarded as a crystal part of InGaZnO₄. Each of lattice fringescorresponds to the a-b plane of the InGaZnO₄ crystal.

FIG. 28 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 28 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 28, acrystal part of approximately 1.2 nm at the start of TEM observationgrows to a size of approximately 2.6 nm at a cumulative electron dose of4.2×10⁸ e⁻/nm². In contrast, the crystal part size in the nc-OS and theCAAC-OS shows little change from the start of electron irradiation to acumulative electron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by(2) and (3) in FIG. 28, the average crystal sizes in an nc-OS and aCAAC-OS are approximately 1.4 nm and approximately 2.1 nm, respectively,regardless of the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit contains a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having adesired composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more films of an amorphous oxide semiconductor,an a-like OS, a microcrystalline oxide semiconductor, and a CAAC-OS, forexample.

The semiconductor device of one embodiment of the present invention canbe formed using an oxide semiconductor film having any of the abovestructures.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 4

In this embodiment, a display device that includes a semiconductordevice of one embodiment of the present invention is described withreference to FIGS. 29A to 29C.

<4. Display Device>

The display device illustrated in FIG. 29A includes a region includingpixels of display elements (hereinafter the region is referred to as apixel portion 502), a circuit portion provided outside the pixel portion502 and including a circuit for driving the pixels (hereinafter theportion is referred to as a driver circuit portion 504), circuits eachhaving a function of protecting an element (hereinafter the circuits arereferred to as protection circuits 506), and a terminal portion 507.Note that the protection circuits 506 are not necessarily provided.

A part or the whole of the driver circuit portion 504 is preferablyformed over a substrate over which the pixel portion 502 is formed, inwhich case the number of components and the number of terminals can bereduced. When a part or the whole of the driver circuit portion 504 isnot formed over the substrate over which the pixel portion 502 isformed, the part or the whole of the driver circuit portion 504 can bemounted by COG or tape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for drivingdisplay elements arranged in X rows (X is a natural number of 2 or more)and Y columns (Y is a natural number of 2 or more) (hereinafter, suchcircuits are referred to as pixel circuits 501). The driver circuitportion 504 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (hereinafter, the circuit isreferred to as a gate driver 504 a) and a circuit for supplying a signal(data signal) to drive a display element in a pixel (hereinafter, thecircuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gatedriver 504 a receives a signal for driving the shift register throughthe terminal portion 507 and outputs a signal. For example, the gatedriver 504 a receives a start pulse signal, a clock signal, or the likeand outputs a pulse signal. The gate driver 504 a has a function ofcontrolling the potentials of wirings supplied with scan signals(hereinafter, such wirings are referred to as scan lines GL_1 to GL_X).Note that a plurality of gate drivers 504 a may be provided to controlthe scan lines GL_1 to GL_X separately. Alternatively, the gate driver504 a has a function of supplying an initialization signal. Withoutbeing limited thereto, the gate driver 504 a can supply another signal.

The source driver 504 b includes a shift register or the like. Thesource driver 504 b receives a signal (video signal) from which a datasignal is derived, as well as a signal for driving the shift register,through the terminal portion 507. The source driver 504 b has a functionof generating a data signal to be written to the pixel circuit 501 whichis based on the video signal. In addition, the source driver 504 b has afunction of controlling output of a data signal in response to a pulsesignal produced by input of a start pulse signal, a clock signal, or thelike. Furthermore, the source driver 504 b has a function of controllingthe potentials of wirings supplied with data signals (hereinafter suchwirings are referred to as data lines DL_1 to DL_Y). Alternatively, thesource driver 504 b has a function of supplying an initializationsignal. Without being limited thereto, the source driver 504 b cansupply another signal.

The source driver 504 b includes a plurality of analog switches or thelike, for example. The source driver 504 b can output, as the datasignals, signals obtained by time-dividing the video signal bysequentially turning on the plurality of analog switches. The sourcedriver 504 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 501 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal to and in each of the plurality of pixel circuits 501 arecontrolled by the gate driver 504 a. For example, to the pixel circuit501 in the m-th row and the n-th column (m is a natural number of lessthan or equal to X, and n is a natural number of less than or equal toY), a pulse signal is input from the gate driver 504 a through the scanline GL_m, and a data signal is input from the source driver 504 bthrough the data line DL_n in accordance with the potential of the scanline GL_m.

The protection circuit 506 shown in FIG. 29A is connected to, forexample, the scan line GL between the gate driver 504 a and the pixelcircuit 501. Alternatively, the protection circuit 506 is connected tothe data line DL between the source driver 504 b and the pixel circuit501. Alternatively, the protection circuit 506 can be connected to awiring between the gate driver 504 a and the terminal portion 507.Alternatively, the protection circuit 506 can be connected to a wiringbetween the source driver 504 b and the terminal portion 507. Note thatthe terminal portion 507 means a portion having terminals for inputtingpower, control signals, and video signals to the display device fromexternal circuits.

The protection circuit 506 is a circuit that electrically connects awiring connected to the protection circuit to another wiring when apotential out of a certain range is applied to the wiring connected tothe protection circuit.

As illustrated in FIG. 29A, the protection circuits 506 are provided forthe pixel portion 502 and the driver circuit portion 504, so that theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 506 is not limited to that, andfor example, the protection circuit 506 may be configured to beconnected to the gate driver 504 a or the protection circuit 506 may beconfigured to be connected to the source driver 504 b. Alternatively,the protection circuit 506 may be configured to be connected to theterminal portion 507.

In FIG. 29A, an example in which the driver circuit portion 504 includesthe gate driver 504 a and the source driver 504 b is shown; however, thestructure is not limited thereto. For example, only the gate driver 504a may be formed and a separately prepared substrate where a sourcedriver circuit is formed (e.g., a driver circuit substrate formed with asingle crystal semiconductor film or a polycrystalline semiconductorfilm) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 29A can have thestructure illustrated in FIG. 29B, for example.

The pixel circuit 501 illustrated in FIG. 29B includes a liquid crystalelement 570, a transistor 550, and a capacitor 560. As the transistor550, any of the transistors described in the above embodiments can beused.

Here, a structure in which the transistor 200 illustrated in FIGS. 11Ato 11C is used as the transistor 550 illustrated in FIG. 29B and anoxide conductive film is used as one electrode of the capacitor 560illustrated in FIG. 29B will be described with reference to FIG. 46.FIG. 46 is a cross-sectional view illustrating an example of a displaydevice.

A display device 500 illustrated in FIG. 46 includes the liquid crystalelement 570, the transistor 550, and the capacitor 560. A conductivefilm 520 functioning as a pixel electrode is electrically connected toone of a source electrode and a drain electrode of the transistor 550.One electrode of the capacitor 560 is an oxide conductive film 528formed by processing the same oxide semiconductor film as the oxidesemiconductor film of the transistor 550, and the other electrode of thecapacitor 560 is the conductive film 520 functioning as a pixelelectrode. Furthermore, the capacitor 560 includes a nitride insulatingfilm 518 between the oxide conductive film 528 and the conductive film520. The nitride insulating film 518 has a function of a protective filmfor the transistor 550 and a dielectric layer of the capacitor 560.

The oxide conductive film 528 is formed by processing the same oxidesemiconductor film as the oxide semiconductor film of the transistor550. Hydrogen is supplied from the nitride insulating film 518 incontact with the oxide conductive film 528 to increase the conductivityof the oxide semiconductor film. As a result, the oxide conductive film528 can function as one electrode of the capacitor 560.

Moreover, the liquid crystal element 570 includes the conductive film520, a liquid crystal layer 546, and a conductive film 540. An alignmentfilm 544 a is provided between the conductive film 520 and the liquidcrystal layer 546, and an alignment film 544 b is provided between theconductive film 540 and the liquid crystal layer 546. An insulating film533, a light-blocking layer 531, a coloring layer 532, and the like areprovided over the conductive film 540. A spacer 542 that controls thethickness of the liquid crystal layer 546, i.e., the cell gap, isprovided over the transistor 550. By an application of a voltage betweenthe conductive film 520 that is one electrode of the liquid crystalelement 570 and the conductive film 540 that is the other electrode ofthe liquid crystal element 570, the alignment state of the liquidcrystal layer 546 can be changed.

The potential of one of a pair of electrodes of the liquid crystalelement 570 is set in accordance with the specifications of the pixelcircuit 501 as appropriate. The alignment state of the liquid crystalelement 570 depends on written data. A common potential may be suppliedto one of the pair of electrodes of the liquid crystal element 570included in each of the plurality of pixel circuits 501. Furthermore,the potential supplied to one of the pair of electrodes of the liquidcrystal element 570 in the pixel circuit 501 in one row may be differentfrom the potential supplied to one of the pair of electrodes of theliquid crystal element 570 in the pixel circuit 501 in another row.

As a driving method of the display device including the liquid crystalelement 570, any of the following modes can be used, for example: atwisted nematic (TN) mode, a super-twisted nematic (STN) mode, avertical alignment (VA) mode, a multi-domain vertical alignment (MVA)mode, a patterned vertical alignment (PVA) mode, an in-plane-switching(IPS) mode, a fringe field switching (FFS) mode, an axially symmetricaligned micro-cell (ASM) mode, an optically compensated birefringence(OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, an FFS mode, a transversebend alignment (TBA) mode, and the like.

Other examples of the driving method of the display device include anelectrically controlled birefringence (ECB) mode, a polymer dispersedliquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC)mode, and a guest-host mode. Note that the present invention is notlimited to these examples, and various liquid crystal elements anddriving methods can be applied to the liquid crystal element and thedriving method thereof.

In the pixel circuit 501 in the m-th row and the n-th column, one of asource electrode and a drain electrode of the transistor 550 iselectrically connected to the data line DL_n, and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. A gate electrode of the transistor 550 iselectrically connected to the scan line GL_m. The transistor 550 has afunction of controlling whether to write a data signal by being turnedon or off.

One of a pair of electrodes of the capacitor 560 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL), and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. The potential of the potential supply lineVL is set in accordance with the specifications of the pixel circuit 501as appropriate. The capacitor 560 functions as a storage capacitor forstoring written data.

For example, in the display device including the pixel circuit 501 inFIG. 29B, the pixel circuits 501 are sequentially selected row by row bythe gate driver 504 a illustrated in FIG. 29A, whereby the transistors550 are turned on and a data signal is written.

When the transistors 550 are turned off, the pixel circuits 501 in whichthe data has been written are brought into a holding state. Thisoperation is sequentially performed row by row; thus, an image can bedisplayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 29Acan have the structure illustrated in FIG. 29C, for example.

The pixel circuit 501 illustrated in FIG. 29C includes transistors 552and 554, a capacitor 562, and a light-emitting element 572. Any of thetransistors described in the above embodiments can be used as one orboth of the transistors 552 and 554.

One of a source electrode and a drain electrode of the transistor 552 iselectrically connected to a wiring to which a data signal is supplied(hereinafter referred to as a signal line DL_n). A gate electrode of thetransistor 552 is electrically connected to a wiring to which a gatesignal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 552 has a function of controlling whether to write a datasignal by being turned on or off.

One of a pair of electrodes of the capacitor 562 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 552.

The capacitor 562 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 554 iselectrically connected to the potential supply line VL_a. Furthermore, agate electrode of the transistor 554 is electrically connected to theother of the source electrode and the drain electrode of the transistor552.

One of an anode and a cathode of the light-emitting element 572 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 572 is not limited to anorganic EL element; an inorganic EL element including an inorganicmaterial may be used.

A high power supply potential VDD is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential VSS is supplied to the other.

For example, in the display device including the pixel circuit 501 inFIG. 29C, the pixel circuits 501 are sequentially selected row by row bythe gate driver 504 a illustrated in FIG. 29A, whereby the transistors552 are turned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 501 in whichthe data has been written are brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 554 is controlled in accordance with thepotential of the written data signal. The light-emitting element 572emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage can be displayed.

Although the structures including the liquid crystal element 570 or thelight-emitting element 572 as a display element of the display deviceare described in this embodiment, one embodiment of the presentinvention is not limited to these structures and a variety of elementsmay be included in the display device.

For example, the display device may include a display medium whosecontrast, luminance, reflectivity, transmittance, or the like is changedby electrical or magnetic effect, such as a liquid crystal element, anEL element (e.g., an EL element including organic and inorganicmaterials, an organic EL element, or an inorganic EL element), an LED(e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor(a transistor that emits light depending on current), an electronemitter, electronic ink, an electrophoretic element, a grating lightvalve (GLV), a plasma display panel (PDP), a display element using microelectro mechanical system (MEMS), a digital micromirror device (DMD), adigital micro shutter (DMS), MIRASOL (registered trademark), aninterferometric modulator display (IMOD) element, a MEMS shutter displayelement, an optical-interference-type MEMS display element, anelectrowetting element, a piezoelectric ceramic display, and a displayelement using a carbon nanotube. Examples of display devices includingelectron emitters are a field emission display (FED) and an SED-typeflat panel display (SED: surface-conduction electron-emitter display).Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). An example of a display device including electronicink or electrophoretic elements is electronic paper. In the case of atransflective liquid crystal display or a reflective liquid crystaldisplay, some of or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed tocontain aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes. Thus,the power consumption can be further reduced.

A progressive type display, an interlace type display, or the like canbe employed as the display type of the display device of thisembodiment. Further, color elements controlled in a pixel at the time ofcolor display are not limited to three colors: R, G, and B (R, G, and Bcorrespond to red, green, and blue, respectively). For example, fourpixels of the R pixel, the G pixel, the B pixel, and a W (white) pixelmay be included. Alternatively, a color element may be composed of twocolors among R, G, and B as in PenTile layout. The two colors may differamong color elements. Alternatively, one or more colors of yellow, cyan,magenta, and the like may be added to RGB. Further, the size of adisplay region may be different depending on respective dots of thecolor elements. Embodiments of the disclosed invention are not limitedto a display device for color display; the disclosed invention can alsobe applied to a display device for monochrome display.

White light (W) may be emitted from a backlight (e.g., an organic ELelement, an inorganic EL element, an LED, or a fluorescent lamp) in thedisplay device. Furthermore, a coloring layer (also referred to as acolor filter) may be provided in the display device. As the coloringlayer, red (R), green (G), blue (B), yellow (Y), or the like may becombined as appropriate, for example. With the use of the coloringlayer, higher color reproducibility can be obtained than in the casewithout the coloring layer. In this case, by providing a region with thecoloring layer and a region without the coloring layer, white light inthe region without the coloring layer may be directly utilized fordisplay. By partly providing the region without the coloring layer, adecrease in luminance due to the coloring layer can be suppressed, and20% to 30% of power consumption can be reduced in some cases when animage is displayed brightly. Note that in the case where full-colordisplay is performed using self-luminous elements such as organic ELelements or inorganic EL elements, the elements may emit light of theirrespective colors R, G, B, Y, and W. By using self-luminous elements,power consumption can be further reduced as compared to the case ofusing the coloring layer in some cases.

The structures described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, a display device including a semiconductor device ofone embodiment of the present invention and an electronic device inwhich the display device is provided with an input device will bedescribed with reference to FIGS. 30A and 30B, FIGS. 31A and 31B, FIG.32, FIGS. 33A and 33B, FIGS. 34A and 34B, and FIG. 35.

<5-1. Touch Panel>

In this embodiment, a touch panel 2000 including a display device and aninput device will be described as an example of an electronic device. Inaddition, an example in which a touch sensor is used as an input devicewill be described.

FIGS. 30A and 30B are perspective views of the touch panel 2000. Notethat FIGS. 30A and 30B illustrate only main components of the touchpanel 2000 for simplicity.

The touch panel 2000 includes a display device 2501 and a touch sensor2595 (see FIG. 30B). The touch panel 2000 also includes a substrate2510, a substrate 2570, and a substrate 2590. The substrate 2510, thesubstrate 2570, and the substrate 2590 each have flexibility. Note thatone or all of the substrates 2510, 2570, and 2590 may be inflexible.

The display device 2501 includes a plurality of pixels over thesubstrate 2510 and a plurality of wirings 2511 through which signals aresupplied to the pixels. The plurality of wirings 2511 are led to aperipheral portion of the substrate 2510, and parts of the plurality ofwirings 2511 form a terminal 2519. The terminal 2519 is electricallyconnected to an FPC 2509(1).

The substrate 2590 includes the touch sensor 2595 and a plurality ofwirings 2598 electrically connected to the touch sensor 2595. Theplurality of wirings 2598 are led to a peripheral portion of thesubstrate 2590, and parts of the plurality of wirings 2598 form aterminal. The terminal is electrically connected to an FPC 2509(2). Notethat in FIG. 30B, electrodes, wirings, and the like of the touch sensor2595 provided on the back side of the substrate 2590 (the side facingthe substrate 2510) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used.Examples of the capacitive touch sensor are a surface capacitive touchsensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self capacitivetouch sensor and a mutual capacitive touch sensor, which differ mainlyin the driving method. The use of a mutual capacitive type is preferablebecause multiple points can be sensed simultaneously.

Note that the touch sensor 2595 illustrated in FIG. 30B is an example ofusing a projected capacitive touch sensor.

Note that a variety of sensors that can sense proximity or touch of asensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 andelectrodes 2592. The electrodes 2591 are electrically connected to anyof the plurality of wirings 2598, and the electrodes 2592 areelectrically connected to any of the other wirings 2598.

The electrodes 2592 each have a shape of a plurality of quadranglesarranged in one direction with one corner of a quadrangle connected toone corner of another quadrangle as illustrated in FIGS. 30A and 30B.

The electrodes 2591 each have a quadrangular shape and are arranged in adirection intersecting with the direction in which the electrodes 2592extend.

A wiring 2594 electrically connects two electrodes 2591 between whichthe electrode 2592 is positioned. The intersecting area of the electrode2592 and the wiring 2594 is preferably as small as possible. Such astructure allows a reduction in the area of a region where theelectrodes are not provided, reducing variation in transmittance. As aresult, variation in luminance of light passing through the touch sensor2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 arenot limited thereto and can be any of a variety of shapes. For example,a structure may be employed in which the plurality of electrodes 2591are arranged so that gaps between the electrodes 2591 are reduced asmuch as possible, and the electrodes 2592 are spaced apart from theelectrodes 2591 with an insulating layer interposed therebetween to haveregions not overlapping with the electrodes 2591. In this case, it ispreferable to provide, between two adjacent electrodes 2592, a dummyelectrode electrically insulated from these electrodes because the areaof regions having different transmittances can be reduced.

Note that as a material of the conductive films such as the electrodes2591, the electrodes 2592, and the wirings 2598, that is, wirings andelectrodes forming the touch panel, a transparent conductive filmincluding indium oxide, tin oxide, zinc oxide, or the like (e.g., ITO)can be given. For example, a low-resistance material is preferably usedas a material that can be used as the wirings and electrodes forming thetouch panel. For example, silver, copper, aluminum, a carbon nanotube,graphene, or a metal halide (such as a silver halide) may be used.Alternatively, a metal nanowire including a plurality of conductors withan extremely small width (for example, a diameter of several nanometers)may be used. Further alternatively, a net-like metal mesh with aconductor may be used. For example, an Ag nanowire, a Cu nanowire, an Alnanowire, an Ag mesh, a Cu mesh, or an Al mesh may be used. For example,in the case of using an Ag nanowire as the wirings and electrodesforming the touch panel, a visible light transmittance of 89% or moreand a sheet resistance of 40 Ω/cm² or more and 100 Ω/cm² or less can beachieved. Since the above-described metal nanowire, metal mesh, carbonnanotube, graphene, and the like, which are examples of the materialthat can be used as the wirings and electrodes forming the touch panel,have high visible light transmittances, they may be used as electrodesof display elements (e.g., a pixel electrode or a common electrode).

<5-2. Display Device>

Next, the display device 2501 will be described in detail with referenceto FIGS. 31A and 31B. FIGS. 31A and 31B correspond to cross-sectionalviews taken along dashed-dotted line X1-X2 in FIG. 30B.

The display device 2501 includes a plurality of pixels arranged in amatrix. Each of the pixels includes a display element and a pixelcircuit for driving the display element.

(Structure with an EL Element as a Display Element)

First, a structure that uses an EL element as a display element will bedescribed below with reference to FIG. 31A. In the followingdescription, an example of using an EL element that emits white lightwill be described; however, the EL element is not limited to thiselement. For example, EL elements that emit light of different colorsmay be included so that the light of different colors can be emittedfrom adjacent pixels.

For the substrate 2510 and the substrate 2570, for example, a flexiblematerial with a vapor permeability of lower than or equal to 10⁻⁵g/(m²·day), preferably lower than or equal to 10⁻⁶ g/(m²·day) can befavorably used. Alternatively, materials whose thermal expansioncoefficients are substantially equal to each other are preferably usedfor the substrate 2510 and the substrate 2570. For example, thecoefficients of linear expansion of the materials are preferably lowerthan or equal to 1×10⁻³/K, further preferably lower than or equal to5×10⁻⁵/K, and still further preferably lower than or equal to 1×10⁻⁵/K.

Note that the substrate 2510 is a stacked body including an insulatinglayer 2510 a for preventing impurity diffusion into the EL element, aflexible substrate 2510 b, and an adhesive layer 2510 c for attachingthe insulating layer 2510 a and the flexible substrate 2510 b to eachother. The substrate 2570 is a stacked body including an insulatinglayer 2570 a for preventing impurity diffusion into the EL element, aflexible substrate 2570 b, and an adhesive layer 2570 c for attachingthe insulating layer 2570 a and the flexible substrate 2570 b to eachother.

For the adhesive layer 2510 c and the adhesive layer 2570 c, forexample, materials that include polyester, polyolefin, polyamide (e.g.,nylon, aramid), polyimide, polycarbonate, polyurethane, an acrylicresin, an epoxy resin, or a resin having a siloxane bond can be used.

A sealing layer 2560 is provided between the substrate 2510 and thesubstrate 2570. The sealing layer 2560 preferably has a refractive indexhigher than that of air. In the case where light is extracted to thesealing layer 2560 side as illustrated in FIG. 31A, the sealing layer2560 can also serve as an optical layer.

A sealant may be formed in the peripheral portion of the sealing layer2560. With the use of the sealant, an EL element 2550 can be provided ina region surrounded by the substrate 2510, the substrate 2570, thesealing layer 2560, and the sealant. Note that an inert gas (such asnitrogen or argon) may be used instead of the sealing layer 2560. Adrying agent may be provided in the inert gas so as to adsorb moistureor the like. For example, an epoxy-based resin or a glass frit ispreferably used as the sealant. As a material used for the sealant, amaterial which is impermeable to moisture or oxygen is preferably used.

The display device 2501 illustrated in FIG. 31A includes a pixel 2505.The pixel 2505 includes a light-emitting module 2580, the EL element2550 and a transistor 2502 t that can supply electric power to the ELelement 2550. Note that the transistor 2502 t functions as part of thepixel circuit.

The light-emitting module 2580 includes the EL element 2550 and acoloring layer 2567. The EL element 2550 includes a lower electrode, anupper electrode, and an EL layer between the lower electrode and theupper electrode.

In the case where the sealing layer 2560 is provided on the lightextraction side, the sealing layer 2560 is in contact with the ELelement 2550 and the coloring layer 2567.

The coloring layer 2567 is positioned in a region overlapping with theEL element 2550. Accordingly, part of light emitted from the EL element2550 passes through the coloring layer 2567 and is emitted to theoutside of the light-emitting module 2580 as indicated by an arrow inFIG. 31A.

The display device 2501 includes a light-blocking layer 2568 on thelight extraction side. The light-blocking layer 2568 is provided so asto surround the coloring layer 2567.

The coloring layer 2567 is a coloring layer having a function oftransmitting light in a particular wavelength region. For example, acolor filter for transmitting light in a red wavelength range, a colorfilter for transmitting light in a green wavelength range, a colorfilter for transmitting light in a blue wavelength range, a color filterfor transmitting light in a yellow wavelength range, or the like can beused. Each color filter can be formed with any of various materials by aprinting method, an inkjet method, an etching method using aphotolithography technique, or the like.

An insulating layer 2521 is provided in the display device 2501. Theinsulating layer 2521 covers the transistor 2502 t and the like. Notethat the insulating layer 2521 has a function of covering the roughnesscaused by the pixel circuit to provide a flat surface. The insulatinglayer 2521 may have a function of suppressing impurity diffusion. Thiscan prevent the reliability of the transistor 2502 t or the like frombeing lowered by impurity diffusion.

The EL element 2550 is formed over the insulating layer 2521. Apartition 2528 is provided so as to overlap with an end portion of thelower electrode of the EL element 2550. Note that a spacer forcontrolling the distance between the substrate 2510 and the substrate2570 may be formed over the partition 2528.

A scan line driver circuit 2504 includes a transistor 2503 t and acapacitor 2503 c. Note that the driver circuit can be formed in the sameprocess and over the same substrate as those of the pixel circuits.

The wirings 2511 through which signals can be supplied are provided overthe substrate 2510. The terminal 2519 is provided over the wirings 2511.The FPC 2509(1) is electrically connected to the terminal 2519. The FPC2509(1) has a function of supplying a video signal, a clock signal, astart signal, a reset signal, or the like. Note that the FPC 2509(1) maybe provided with a printed wiring board (PWB).

Any of the transistors described in the above embodiments may be used asone or both of the transistors 2502 t and 2503 t. The transistors usedin this embodiment each include an oxide semiconductor film which ishighly purified and in which formation of oxygen vacancies issuppressed. In the transistors, the current in an off state (off-statecurrent) can be made small. Accordingly, an electrical signal such as animage signal can be held for a longer period, and a writing interval canbe set longer in an on state. Accordingly, the frequency of refreshoperation can be reduced, which leads to an effect of suppressing powerconsumption. In addition, the transistors used in this embodiment canhave relatively high field-effect mobility and thus are capable of highspeed operation. For example, with such transistors which can operate athigh speed used for the display device 2501, a switching transistor of apixel circuit and a driver transistor in a driver circuit portion can beformed over one substrate. That is, a semiconductor device formed usinga silicon wafer or the like is not additionally needed as a drivercircuit, by which the number of components of the semiconductor devicecan be reduced. In addition, by using a transistor which can operate athigh speed in a pixel circuit, a high-quality image can be provided.

(Structure with a Liquid Crystal Element as a Display Element)

Next, a structure including a liquid crystal element as a displayelement is described below with reference to FIG. 31B. In thedescription below, a reflective liquid crystal display device thatperforms display by reflecting external light is described; however, oneembodiment of the present invention is not limited to this type ofliquid crystal display device. For example, a light source (e.g., a backlight or a side light) may be provided to form a transmissive liquidcrystal display device or a transflective liquid crystal display device.

The display device 2501 illustrated in FIG. 31B has the same structureas the display device 2501 illustrated in FIG. 31A except the followingpoints.

The pixel 2505 in the display device 2501 illustrated in FIG. 31Bincludes a liquid crystal element 2551 and the transistor 2502 t thatcan supply electric power to the liquid crystal element 2551.

The liquid crystal element 2551 includes a lower electrode (alsoreferred to as a pixel electrode), an upper electrode, and a liquidcrystal layer 2529 between the lower electrode and the upper electrode.By the application of a voltage between the lower electrode and theupper electrode, the alignment state of the liquid crystal layer 2529 inthe liquid crystal element 2551 can be changed. Furthermore, in theliquid crystal layer 2529, a spacer 2530 a and a spacer 2530 b areprovided. Although not illustrated in FIG. 31B, an alignment film may beprovided on each of the upper electrode and the lower electrode on theside in contact with the liquid crystal layer 2529.

As the liquid crystal layer 2529, thermotropic liquid crystal,low-molecular liquid crystal, high-molecular liquid crystal, polymerdispersed liquid crystal, ferroelectric liquid crystal, oranti-ferroelectric liquid crystal can be used. Such a liquid crystalmaterial exhibits a cholesteric phase, a smectic phase, a cubic phase, achiral nematic phase, an isotropic phase, or the like depending onconditions. In the case of employing a horizontal electric field modeliquid crystal display device, liquid crystal exhibiting a blue phasefor which an alignment film is unnecessary may be used. In the casewhere a liquid crystal exhibiting a blue phase is used, an alignmentfilm is not necessarily provided, so that rubbing treatment is alsounnecessary. Accordingly, electrostatic discharge damage caused by therubbing treatment can be prevented and defects and damage of the liquidcrystal display device in the manufacturing process can be reduced.

The spacers 2530 a and 2530 b are formed by selectively etching aninsulating film. The spacers 2530 a and 2530 b are provided in order tocontrol the distance between the substrate 2510 and the substrate 2570(the cell gap). Note that the spacers 2530 a and 2530 b may havedifferent sizes from each other and are preferably have a columnar orspherical shape. Although the spacers 2530 a and 2530 b are provided onthe substrate 2570 side in the non-limiting structure in FIG. 31B, theymay be provided on the substrate 2510 side.

The upper electrode of the liquid crystal element 2551 is provided onthe substrate 2570 side. An insulating layer 2531 is provided betweenthe upper electrode and the coloring layer 2567 and the light-blockinglayer 2568. The insulating layer 2531 has a function of covering theroughness caused by the coloring layer 2567 and the light-blocking layer2568 to provide a flat surface. As the insulating layer 2531, an organicresin film may be used, for example. The lower electrode of the liquidcrystal element 2551 has a function of a reflective electrode. Thedisplay device 2501 illustrated in FIG. 31B is of a reflective typewhich performs display by reflecting external light at the lowerelectrode and making the light pass through the coloring layer 2567.Note that in the case of forming a transmissive liquid crystal displaydevice, a transparent electrode is provided as the lower electrode.

The display device 2501 illustrated in FIG. 31B includes an insulatinglayer 2522. The insulating layer 2522 covers the transistor 2502 t andthe like. The insulating layer 2522 has a function of covering theroughness caused by the pixel circuit to provide a flat surface and afunction of forming roughness on the lower electrode of the liquidcrystal element. In this way, roughness can be formed on the surface ofthe lower electrode. Therefore, when external light is incident on thelower electrode, the light is reflected diffusely at the surface of thelower electrode, whereby visibility can be improved. Note that in thecase of forming a transmissive liquid crystal display device, astructure without such roughness may be employed.

<5-3. Touch Sensor>

Next, the touch sensor 2595 will be described in detail with referenceto FIG. 32. FIG. 32 corresponds to a cross-sectional view taken alongdashed-dotted line X3-X4 in FIG. 30B.

The touch sensor 2595 includes the electrodes 2591 and the electrodes2592 provided in a staggered arrangement on the substrate 2590, aninsulating layer 2593 covering the electrodes 2591 and the electrodes2592, and the wiring 2594 that electrically connects the adjacentelectrodes 2591 to each other.

The electrodes 2591 and the electrodes 2592 are formed using alight-transmitting conductive material. As a light-transmittingconductive material, a conductive oxide such as indium oxide, indium tinoxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium isadded can be used. Note that a film including graphene may be used aswell. The film including graphene can be formed, for example, byreducing a film containing graphene oxide. As a reducing method, amethod with application of heat or the like can be employed.

The electrodes 2591 and the electrodes 2592 may be formed by, forexample, depositing a light-transmitting conductive material on thesubstrate 2590 by a sputtering method and then removing an unnecessaryportion by any of various patterning techniques such asphotolithography.

Examples of a material for the insulating layer 2593 are a resin such asan acrylic resin or an epoxy resin, a resin having a siloxane bond, andan inorganic insulating material such as silicon oxide, siliconoxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer2593, and the wiring 2594 electrically connects the adjacent electrodes2591. A light-transmitting conductive material can be favorably used asthe wiring 2594 because the aperture ratio of the touch panel can beincreased. Moreover, a material with higher conductivity than theconductivities of the electrodes 2591 and 2592 can be favorably used forthe wiring 2594 because electric resistance can be reduced.

One electrode 2592 extends in one direction, and a plurality ofelectrodes 2592 are provided in the form of stripes. The wiring 2594intersects with the electrode 2592.

Adjacent electrodes 2591 are provided with one electrode 2592 providedtherebetween. The wiring 2594 electrically connects the adjacentelectrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arrangedin the direction orthogonal to one electrode 2592 and may be arranged tointersect with one electrode 2592 at an angle of more than 0 degrees andless than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591and 2592. Part of the wiring 2598 functions as a terminal. For thewiring 2598, a metal material such as aluminum, gold, platinum, silver,nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper,or palladium or an alloy material containing any of these metalmaterials can be used.

Note that an insulating layer that covers the insulating layer 2593 andthe wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC2509(2).

As the connection layer 2599, any of various anisotropic conductivefilms (ACF), anisotropic conductive pastes (ACP), or the like can beused.

<5-4. Touch Panel>

Next, the touch panel 2000 will be described in detail with reference toFIG. 33A. FIG. 33A corresponds to a cross-sectional view taken alongdashed-dotted line X5-X6 in FIG. 30A.

In the touch panel 2000 illustrated in FIG. 33A, the display device 2501described with reference to FIG. 31A and the touch sensor 2595 describedwith reference to FIG. 32 are attached to each other.

The touch panel 2000 illustrated in FIG. 33A includes an adhesive layer2597 and an anti-reflective layer 2569 in addition to the componentsdescribed with reference to FIG. 31A.

The adhesive layer 2597 is provided in contact with the wiring 2594.Note that the adhesive layer 2597 attaches the substrate 2590 to thesubstrate 2570 so that the touch sensor 2595 overlaps with the displaydevice 2501. The adhesive layer 2597 preferably has a light-transmittingproperty. A heat curable resin or an ultraviolet curable resin can beused for the adhesive layer 2597. For example, an acrylic resin, aurethane-based resin, an epoxy-based resin, or a siloxane-based resincan be used.

The anti-reflective layer 2569 is positioned in a region overlappingwith pixels. As the anti-reflective layer 2569, a circularly polarizingplate can be used, for example.

Next, a touch panel having a structure different from that illustratedin FIG. 33A will be described with reference to FIG. 33B.

FIG. 33B is a cross-sectional view of a touch panel 2001. The touchpanel 2001 illustrated in FIG. 33B differs from the touch panel 2000illustrated in FIG. 33A in the position of the touch sensor 2595relative to the display device 2501. Different parts are described indetail below, and the above description of the touch panel 2000 isreferred to for the other similar parts.

The coloring layer 2567 is positioned under the EL element 2550. The ELelement 2550 illustrated in FIG. 33B emits light to the side where thetransistor 2502 t is provided. Accordingly, part of light emitted fromthe EL element 2550 passes through the coloring layer 2567 and isemitted to the outside of the light-emitting module 2580 as indicated byan arrow in FIG. 33B.

The touch sensor 2595 is provided on the substrate 2510 side of thedisplay device 2501.

The adhesive layer 2597 is provided between the substrate 2510 and thesubstrate 2590 and attaches the touch sensor 2595 to the display device2501.

As illustrated in FIG. 33A or FIG. 33B, light may be emitted from thelight-emitting element to one or both of upper and lower sides of thesubstrate.

<5-5. Driving Method of Touch Panel>

Next, an example of a method for driving a touch panel will be describedwith reference to FIGS. 34A and 34B.

FIG. 34A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 34A illustrates a pulse voltage outputcircuit 2601 and a current sensing circuit 2602. Note that in FIG. 34A,six wirings X1 to X6 represent the electrodes 2621 to which a pulsevoltage is applied, and six wirings Y1 to Y6 represent the electrodes2622 that detect changes in current. FIG. 34A also illustratescapacitors 2603 that are each formed in a region where the electrodes2621 and 2622 overlap with each other. Note that functional replacementbetween the electrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentiallyapplying a pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 2621 and 2622 of the capacitor 2603. When theelectric field between the electrodes is shielded, for example, a changeoccurs in the capacitor 2603 (mutual capacitance). The approach orcontact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for detecting changes incurrent flowing through the wirings Y1 to Y6 that are caused by thechange in mutual capacitance in the capacitor 2603. No change in currentvalue is detected in the wirings Y1 to Y6 when there is no approach orcontact of a sensing target, whereas a decrease in current value isdetected when mutual capacitance is decreased owing to the approach orcontact of a sensing target. Note that an integrator circuit or the likeis used for sensing of current values.

FIG. 34B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 34A. In FIG. 34B,sensing of a sensing target is performed in all the rows and columns inone frame period. FIG. 34B shows a period when a sensing target is notsensed (not touched) and a period when a sensing target is sensed(touched). Sensed current values of the wirings Y1 to Y6 are shown asthe waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltage. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change in accordance with changes inthe voltages of the wirings X1 to X6. The current value is decreased atthe point of approach or contact of a sensing target and accordingly thewaveform of the voltage value changes.

By detecting a change in mutual capacitance in this manner, the approachor contact of a sensing target can be sensed.

<5-6. Sensor Circuit>

Although FIG. 34A illustrates a passive type touch sensor in which onlythe capacitor 2603 is provided at the intersection of wirings as a touchsensor, an active type touch sensor including a transistor and acapacitor may be used. FIG. 35 illustrates an example of a sensorcircuit included in an active type touch sensor.

The sensor circuit in FIG. 35 includes the capacitor 2603 andtransistors 2611, 2612, and 2613.

A signal G2 is input to a gate of the transistor 2613. A voltage VRES isapplied to one of a source and a drain of the transistor 2613, and oneelectrode of the capacitor 2603 and a gate of the transistor 2611 areelectrically connected to the other of the source and the drain of thetransistor 2613. One of a source and a drain of the transistor 2611 iselectrically connected to one of a source and a drain of the transistor2612, and a voltage VSS is applied to the other of the source and thedrain of the transistor 2611. A signal G1 is input to a gate of thetransistor 2612, and a wiring ML is electrically connected to the otherof the source and the drain of the transistor 2612. The voltage VSS isapplied to the other electrode of the capacitor 2603.

Next, the operation of the sensor circuit in FIG. 35 will be described.First, a potential for turning on the transistor 2613 is supplied as thesignal G2, and a potential with respect to the voltage VRES is thusapplied to the node n connected to the gate of the transistor 2611.Then, a potential for turning off the transistor 2613 is applied as thesignal G2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor 2603 changes owing to theapproach or contact of a sensing target such as a finger, andaccordingly the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor 2612 issupplied as the signal G1. A current flowing through the transistor2611, that is, a current flowing through the wiring ML is changed inaccordance with the potential of the node n. By sensing this current,the approach or contact of a sensing target can be sensed.

In each of the transistors 2611, 2612, and 2613, any of the transistorsdescribed in the above embodiments can be used. In particular, it ispreferable to use any of the transistors described in the aboveembodiments as the transistor 2613 because the potential of the node ncan be held for a long time and the frequency of operation ofresupplying VRES to the node n (refresh operation) can be reduced.

The structures described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 6

In this embodiment, a display module and electronic devices that includea semiconductor device of one embodiment of the present invention aredescribed with reference to FIG. 36 and FIGS. 37A to 37G.

<6-1. Display Module>

In a display module 8000 illustrated in FIG. 36, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight 8007, a frame 8009, a printed board 8010, and a battery 8011are provided between an upper cover 8001 and a lower cover 8002.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and can be formed to overlap the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 to form an optical touch panel.

The backlight 8007 includes light sources 8008. Note that although astructure in which the light sources 8008 are provided over thebacklight 8007 is illustrated in FIG. 36, one embodiment of the presentinvention is not limited to this structure. For example, a structure inwhich the light sources 8008 are provided at an end portion of thebacklight 8007 and a light diffusion plate is further provided may beemployed. Note that the backlight 8007 need not be provided in the casewhere a self-luminous light-emitting element such as an organic ELelement is used or in the case where a reflective panel or the like isemployed.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<6-2. Electronic Device>

FIGS. 37A to 37G illustrate electronic devices. These electronic devicescan each include a housing 9000, a display portion 9001, a speaker 9003,an operation key 9005 (including a power switch or an operation switch),a connection terminal 9006, a sensor 9007 (a sensor having a function ofmeasuring force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,current, voltage, power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), a microphone 9008, and the like.

The electronic devices illustrated in FIGS. 37A to 37G can have avariety of functions, for example, a function of displaying a variety ofinformation (a still image, a moving image, a text image, and the like)on the display portion, a touch panel function, a function of displayinga calendar, the date, the time, and the like, a function of controllingprocessing with a variety of software (programs), a wirelesscommunication function, a function of being connected to a variety ofcomputer networks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a storage medium and displaying the program or data on the displayportion, and the like. Note that functions of the electronic devicesillustrated in FIGS. 37A to 37G are not limited thereto, and theelectronic devices can have a variety of functions. Although notillustrated in FIGS. 37A to 37G, the electronic devices may each have aplurality of display portions. The electronic devices may each have acamera or the like and a function of taking a still image, a function oftaking a moving image, a function of storing the taken image in astorage medium (an external storage medium or a storage mediumincorporated in the camera), a function of displaying the taken image onthe display portion, and the like.

The electronic devices illustrated in FIGS. 37A to 37G will be describedin detail below.

FIG. 37A is a perspective view of a portable information terminal 9100.The display portion 9001 of the portable information terminal 9100 isflexible and thus can be incorporated along the curved surface of thehousing 9000. Furthermore, the display portion 9001 includes a touchsensor, and operation can be performed by touching a screen with afinger, a stylus, or the like. For example, by touching an icondisplayed on the display portion 9001, an application can be started.

FIG. 37B is a perspective view of a portable information terminal 9101.The portable information terminal 9101 functions as, for example, one ormore of a telephone set, a notebook, an information browsing system, andthe like. Specifically, the portable information terminal 9101 can beused as a smartphone. Note that the speaker 9003, the connectionterminal 9006, the sensor 9007, and the like, which are not illustratedin FIG. 37B, can be positioned in the portable information terminal 9101as in the portable information terminal 9100 illustrated in FIG. 37A.The portable information terminal 9101 can display characters and imageinformation on its plurality of surfaces. For example, three operationbuttons 9050 (also referred to as operation icons, or simply, icons) canbe displayed on one surface of the display portion 9001. Furthermore,information 9051 indicated by dashed rectangles can be displayed onanother surface of the display portion 9001. Examples of the information9051 include notification from a social networking service (SNS),display indicating reception of an e-mail or an incoming call, the titleof the e-mail, the SNS, or the like, the sender of the e-mail, the SNS,or the like, the date, the time, remaining battery, and the receptionstrength of an antenna. Instead of the information 9051, the operationbuttons 9050 or the like may be displayed in the position where theinformation 9051 is displayed.

FIG. 37C is a perspective view of a portable information terminal 9102.The portable information terminal 9102 has a function of displayinginformation on three or more surfaces of the display portion 9001. Here,information 9052, information 9053, and information 9054 are displayedon different surfaces. For example, a user of the portable informationterminal 9102 can see the display (here, the information 9053) with theportable information terminal 9102 put in a breast pocket of his/herclothes. Specifically, a caller's phone number, name, or the like of anincoming call is displayed in the position that can be seen from abovethe portable information terminal 9102. Thus, the user can see thedisplay without taking out the portable information terminal 9102 fromthe pocket and decide whether to answer the call.

FIG. 37D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, viewing and editing texts, music reproduction, Internetcommunication, and computer games. The display surface of the displayportion 9001 is curved, and images can be displayed on the curveddisplay surface. The portable information terminal 9200 can employ nearfield communication conformable to a communication standard. Forexample, hands-free calling can be achieved with mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication. Moreover, the portable information terminal 9200includes the connection terminal 9006, and data can be directlytransmitted to and received from another information terminal via aconnector. Charging through the connection terminal 9006 is alsopossible. Note that the charging operation may be performed by wirelesspower feeding without using the connection terminal 9006.

FIGS. 37E, 37F, and 37G are perspective views of a foldable portableinformation terminal 9201 that is opened, that is shifted from theopened state to the folded state or from the folded state to the openedstate, and that is folded, respectively. The portable informationterminal 9201 is highly portable when folded. When the portableinformation terminal 9201 is opened, a seamless large display regionprovides high browsability. The display portion 9001 of the portableinformation terminal 9201 is supported by three housings 9000 joinedtogether by hinges 9055. By folding the portable information terminal9201 at a connection portion between two housings 9000 with the hinges9055, the portable information terminal 9201 can be reversibly changedin shape from the opened state to the folded state. For example, theportable information terminal 9201 can be bent with a radius ofcurvature of greater than or equal to 1 mm and less than or equal to 150mm.

The electronic devices described in this embodiment each include thedisplay portion for displaying some kinds of information. However, asemiconductor device according to one embodiment of the presentinvention can also be used for an electronic device that does notinclude a display portion. Furthermore, the display portions of theelectronic devices described in this embodiment may also be non-flexibleand can display images on a flat surface without limitation to aflexible mode capable of displaying images on a curved display surfaceor a foldable mode.

The structures described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

EXAMPLE 1

In this example, the hydrogen concentration of an oxide semiconductorfilm included in the semiconductor device of one embodiment of thepresent invention, here, an IGZO film, was measured. Samples A1, A2, A3,and A4 described below were formed for evaluation in this example.

(Sample A1)

In Sample A1, an IGZO film was formed over a glass substrate. As theoxide semiconductor film of Sample A1, a 100-nm-thick IGZO film wasformed with a sputtering apparatus. The IGZO film was deposited underthe conditions where the substrate temperature was 170° C., a depositiongas (Ar/O₂=180/20 sccm (oxygen partial pressure=10%)) was introducedinto a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W wasapplied to a metal oxide sputtering target (In:Ga:Zn=4:2:4.1 in anatomic ratio).

(Sample A2)

In Sample A2, an IGZO film was formed over a glass substrate. As theoxide semiconductor film of Sample A2, a 100-nm-thick IGZO film wasformed with a sputtering apparatus. The IGZO film was deposited underthe conditions where the substrate temperature was 170° C., a depositiongas (Ar/O₂=160/40 sccm (oxygen partial pressure=20%)) was introducedinto a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W wasapplied to a metal oxide sputtering target (In:Ga:Zn=4:2:4.1 in anatomic ratio).

(Sample A3)

In Sample A3, an IGZO film was formed over a glass substrate. As theoxide semiconductor film of Sample A3, a 100-nm-thick IGZO film wasformed with a sputtering apparatus. The IGZO film was deposited underthe conditions where the substrate temperature was 170° C., a depositiongas (Ar/O₂=140/60 sccm (oxygen partial pressure=30%)) was introducedinto a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W wasapplied to a metal oxide sputtering target (In:Ga:Zn=4:2:4.1 in anatomic ratio).

(Sample A4)

In Sample A4, an IGZO film was formed over a glass substrate. As theoxide semiconductor film of Sample A4, a 100-nm-thick IGZO film wasformed with a sputtering apparatus. The IGZO film was deposited underthe conditions where the substrate temperature was 170° C., a depositiongas (Ar/O₂=100/100 sccm (oxygen partial pressure=50%)) was introducedinto a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W wasapplied to a metal oxide sputtering target (In:Ga:Zn=4:2:4.1 in anatomic ratio).

<Evaluation of Hydrogen Concentration in IGZO Film>

The hydrogen concentration in the IGZO films of Samples A1 to A4 wasevaluated. For the evaluation of hydrogen concentration, a SIMS analysisapparatus was used. FIG. 38 shows hydrogen concentration depth profilesof Samples A1 to A4. Furthermore, the hydrogen concentrations in thevicinity of the depth of 50 nm from the surfaces of the IGZO films inSamples A1 to A4 are shown in Table 1.

TABLE 1 Hydrogen concentration Oxygen partial pressure [atoms/cm³]Sample A1 10% 2.64E+19 Sample A2 20% 3.12E+19 Sample A3 30% 3.90E+19Sample A4 50% 4.73E+19

As shown in FIG. 38 and Table 1, when the oxygen partial pressure indeposition of the IGZO film is low, the hydrogen concentration of theIGZO film becomes low.

The structure described in this example can be used as appropriate incombination with any of the structures described in the otherembodiments or the other example.

EXAMPLE 2

In this example, transistors corresponding to the transistor 270B inFIGS. 15C and 15D were formed and tests for electrical characteristicsand reliability were performed. In this example, Samples B1 to B3 andSamples C1 to C3 were formed and used for evaluation. Note that SamplesB1, B2, C1, and C2 are transistors of one embodiment of the presentinvention, and Samples B3 and C3 are transistors for comparison. SamplesB1 to B3 each included ten transistors with a channel length L of 3 μmand a channel width W of 50 μm over a substrate. In addition, Samples C1to C3 each included ten transistors with a channel length L of 6 μm anda channel width W of 50 μm over a substrate.

Samples B1 to B3 and Samples C1 to C3 formed in this example will bedescribed below. Note that the reference numerals used for thetransistor 270B in FIGS. 15C and 15D are used in the followingdescription.

(Samples B1 and C1)

First, the conductive film 204 was formed over the substrate 202. Aglass substrate was used as the substrate 202. As the conductive film204, a 100-nm-thick tungsten film was formed with a sputteringapparatus.

Next, the insulating films 206 and 207 were formed over the substrate202 and the conductive film 204. As the insulating film 206, a400-nm-thick silicon nitride film was formed with a PECVD apparatus. Asthe insulating film 207, a 50-nm-thick silicon oxynitride film wasformed with a PECVD apparatus.

Then, the oxide semiconductor film 208 (the oxide semiconductor film 208b and the oxide semiconductor film 208 c) was formed over the insulatingfilm 207. As the oxide semiconductor film 208 b, a 10-nm-thick IGZO filmwas formed with a sputtering apparatus. The oxide semiconductor film 208b was deposited under the conditions where the substrate temperature was170° C., a deposition gas (Ar/O₂=180/20 sccm (oxygen partialpressure=10%)) was introduced into a chamber, the pressure was 0.6 Pa,and an AC power of 2500 W was applied to a metal oxide sputtering target(In:Ga:Zn=4:2:4.1 in an atomic ratio). As the oxide semiconductor film208 c, a 15-nm-thick IGZO film was formed with a sputtering apparatus.The oxide semiconductor film 208 c was deposited under the conditionswhere the substrate temperature was 170° C., a deposition gas(Ar/O₂=100/100 sccm (oxygen partial pressure=50%)) was introduced into achamber, the pressure was 0.6 Pa, and an AC power of 2500 W was appliedto a metal oxide sputtering target (In:Ga:Zn=1:1:1.2 in an atomicratio).

Next, the conductive films 212 a and 212 b were formed over theinsulating film 207 and the oxide semiconductor film 208. As theconductive films 212 a and 212 b, a 50-nm-thick tungsten film, a400-nm-thick aluminum film, and a 100-nm-thick titanium film weresuccessively formed in vacuum with a sputtering apparatus.

Then, a surface of the oxide semiconductor film 208 c was cleaned. Thecleaning was performed in such a manner that a chemical solutionobtained by diluting an 85% phosphoric acid solution by 100 times wasapplied onto the oxide semiconductor film 208 c and the oxidesemiconductor film 208 c was processed for 15 seconds.

Next, the oxide insulating films 214 a and 214 b were formed over theinsulating film 207, the oxide semiconductor film 208, and theconductive films 212 a and 212 b. As the oxide insulating film 214 a, a50-nm-thick silicon oxynitride film was formed with a PECVD apparatus.As the oxide insulating film 214 b, a 400-nm-thick silicon oxynitridefilm was formed with a PECVD apparatus. Note that the oxide insulatingfilms 214 a and 214 b were formed successively in vacuum with a PECVDapparatus.

The oxide insulating film 214 a was deposited under the conditions wherethe substrate temperature was 220° C., a silane gas at a flow rate of 50sccm and a dinitrogen monoxide gas at a flow rate of 2000 sccm wereintroduced into a chamber, the pressure was 20 Pa, and an RF power of100 W was supplied between parallel-plate electrodes provided in thePECVD apparatus. The oxide insulating film 214 b was deposited under theconditions where the substrate temperature was 220° C., a silane gas ata flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of4000 sccm were introduced into a chamber, the pressure was 200 Pa, andan RF power of 1500 W was supplied between parallel-plate electrodesprovided in the PECVD apparatus.

Then, first heat treatment was performed. The first heat treatment wasperformed at 350° C. in a nitrogen atmosphere for 1 hour.

Next, the protective film 230 was formed over the oxide insulating film214 b. As the protective film 230, a 5-nm-thick ITSO film was used. TheITSO film was deposited under the conditions where the substratetemperature was room temperature, an argon gas at a flow rate of 72 sccmand an oxygen gas at a flow rate of 5 sccm were introduced into achamber, the pressure was 0.15 Pa, and a DC power of 1000 W was suppliedto a metal oxide target (In₂O₃:SnO₂:SiO₂=85:10:5 [wt. %]) provided in asputtering apparatus.

Next, oxygen addition treatment was performed on the oxide insulatingfilms 214 a and 214 b through the protective film 230. The oxygenaddition treatment was performed with an ashing apparatus under theconditions where the substrate temperature was 40° C., an oxygen gas ata flow rate of 250 sccm was introduced into a chamber, the pressure was15 Pa, and an RF power of 4500 W was supplied for 120 sec. betweenparallel-plate electrodes provided in the ashing apparatus so that abias would be applied to the substrate side.

Then, the protective film 230 was removed. The protective film 230 wasremoved using a wet-etching apparatus in such a manner that etching wasperformed using a 5% oxalic acid solution for 300 sec. and then etchingwas performed using a 0.5% hydrofluoric acid for 15 sec.

Next, the nitride insulating film 218 was formed over the oxideinsulating film 214 b. As the nitride insulating film 218, a100-nm-thick silicon nitride film was formed with a PECVD apparatus. Thenitride insulating film 218 was deposited under the conditions where thesubstrate temperature was 350° C.; a silane gas at a flow rate of 50sccm, a nitrogen gas at a flow rate of 5000 sccm, and an ammonia gas ata flow rate of 100 sccm were introduced into a chamber; the pressure was100 Pa; and an RF power of 1000 W was supplied between parallel-plateelectrodes provided in the PECVD apparatus.

Next, the opening 252 c reaching the conductive film 212 b and theopenings 252 a and 252 b reaching the conductive film 204 were formed.The openings 252 a, 252 b, and 252 c were formed with a dry etchingapparatus.

Next, a conductive film was formed over the nitride insulating film 218to cover the openings 252 a, 252 b, and 252 c and processed to form theconductive films 220 a and 220 b. As the conductive films 220 a and 220b, a 100-nm-thick ITSO film was formed with a sputtering apparatus. Thecomposition of a target used for forming the ITSO film was the same asthat used to form the protective film 230.

Then, second heat treatment was performed. The second heat treatment wasperformed at 250° C. for 1 hour in a nitrogen atmosphere.

Through the above process, Samples B1 and C1 were formed.

(Samples B2 and C2)

Samples B2 and C2 are formed through the same steps as those for SamplesB1 and C1 except the following steps.

The oxide semiconductor film 208 (the oxide semiconductor film 208 b andthe oxide semiconductor film 208 c) was formed over the insulating film207. As the oxide semiconductor film 208 b, a 10-nm-thick IGZO film wasformed with a sputtering apparatus. The oxide semiconductor film 208 bwas deposited under the conditions where the substrate temperature was170° C., a deposition gas (Ar/O₂=140/60 sccm (oxygen partialpressure=30%)) was introduced into a chamber, the pressure was 0.6 Pa,and an AC power of 2500 W was applied to a metal oxide sputtering target(In:Ga:Zn=4:2:4.1 in an atomic ratio). As the oxide semiconductor film208 c, a 15-nm-thick IGZO film was formed with a sputtering apparatus.The oxide semiconductor film 208 c was deposited under the conditionswhere the substrate temperature was 170° C., a deposition gas(Ar/O₂=100/100 sccm (oxygen partial pressure=50%)) was introduced into achamber, the pressure was 0.6 Pa, and an AC power of 2500 W was appliedto a metal oxide sputtering target (In:Ga:Zn=1:1:1.2 in an atomicratio).

(Samples B3 and C3)

Samples B3 and C3 are formed through the same steps as those for SamplesB1 and C1 except the following steps.

The oxide semiconductor film 208 (the oxide semiconductor film 208 b andthe oxide semiconductor film 208 c) was formed over the insulating film207. As the oxide semiconductor film 208 b, a 10-nm-thick IGZO film wasformed with a sputtering apparatus. The oxide semiconductor film 208 bwas deposited under the conditions where the substrate temperature was170° C., a deposition gas (Ar/O₂=100/100 sccm (oxygen partialpressure=50%)) was introduced into a chamber, the pressure was 0.6 Pa,and an AC power of 2500 W was applied to a metal oxide sputtering target(In:Ga:Zn=4:2:4.1 in an atomic ratio). As the oxide semiconductor film208 c, a 15-nm-thick IGZO film was formed with a sputtering apparatus.The oxide semiconductor film 208 c was deposited under the conditionswhere the substrate temperature was 170° C., a deposition gas(Ar/O₂=100/100 sccm (oxygen partial pressure=50%)) was introduced into achamber, the pressure was 0.6 Pa, and an AC power of 2500 W was appliedto a metal oxide sputtering target (In:Ga:Zn=1:1:1.2 in an atomicratio).

Through the above-described steps, Samples B1 to B3 and Samples C1 to C3were formed.

<Evaluation of Electrical Characteristics>

Electrical characteristics of Samples B1 to B3 and Samples C1 to C3 wereevaluated. The evaluation results of Samples B1 to B3 are shown in FIGS.39A and 39B and FIG. 40, and the evaluation results of Samples C1 to C3are shown in FIGS. 41A and 41B and FIG. 42.

FIG. 39A shows electrical characteristics of Sample B1, FIG. 39B showselectrical characteristics of Sample B2, and FIG. 40 shows electricalcharacteristics of Sample B3. FIG. 41A shows electrical characteristicsof Sample C1, FIG. 41B shows electrical characteristics of Sample C2,and FIG. 42 shows electrical characteristics of Sample C3.

In FIGS. 39A and 39B, FIG. 40, FIGS. 41A and 41B, and FIG. 42, thevoltage (V_(d)) between the source electrode and the drain electrode wasset at 1 V and 10 V, and V_(g) was applied from −15 V to 20 V atintervals of 0.25 V. Furthermore, the first vertical axis shows thedrain current (I_(d)), the second vertical axis shows the mobility μFE(cm²/Vs) when V_(d) is 10 V, and the horizontal axis shows the gatevoltage (V_(g)). The solid line represents I_(d), the broken linerepresents μFE, and data of ten transistors are shown for each of them.

The results shown in FIGS. 39A and 39B, FIG. 40, FIGS. 41A and 41B, andFIG. 42 have revealed that Samples B1 to B3 and Samples C1 to C3 havesmall variations among the transistors and have favorable risingcharacteristics in the vicinity of 0 V. Samples B1 and B2 of oneembodiment of the present invention have higher mobilities μFE thanComparative Sample B3. Samples C1 and C2 of one embodiment of thepresent invention have higher mobilities μFE than Comparative Sample C3.

Thus, it was confirmed that a transistor with favorable electricalcharacteristics can be obtained by setting the oxygen partial pressureto be lower than 50% in forming an IGZO film used as the oxidesemiconductor film 208 b.

<Evaluation of Reliability Based on GBT Test>

Next, reliabilities of Samples C1 to C3 were evaluated. For thereliability evaluation, a bias-temperature stress test (hereinafter,referred to as gate bias temperature (GBT) test) was employed.

Note that the GBT test is one kind of accelerated test and a change incharacteristics, caused by long-term usage, of transistors can beevaluated in a short time. In particular, the amount of shift inthreshold voltage (ΔV_(th)) of the transistor between before and afterthe GBT test is an important indicator for examining the reliability.The smaller the shift in the threshold voltage (ΔV_(th)) between beforeand after the GBT test is, the higher the reliability of the transistoris.

The GBT tests in this example were performed under the conditions wherethe gate voltage (V_(g)) was ±30 V; the drain voltage (V_(d)) and thesource voltage (V_(s)) were 0 V (COMMON); the stress temperature was 60°C.; the time for stress application was 1 hour; and two kinds ofmeasurement environments, a dark environment and a photo environment(irradiation with light having approximately 10000 lx with a white LED),were employed. In other words, the source electrode and the drainelectrode of the transistor were set at the same potential, and apotential different from that of the source and drain electrodes wasapplied to the gate electrode for a certain time (one hour, here). Acase where the potential applied to the gate electrode is higher thanthat of the source and drain electrodes is called positive stress, and acase where the potential applied to the gate electrode is lower thanthat of the source and drain electrodes is called negative stress.Therefore, in combination with the measurement environments, the GBTstress test was performed under four stress conditions: dark positivestress, dark negative stress, photo positive stress, and photo negativestress.

FIG. 43A shows GBT test results of Samples C1 to C3. In FIG. 43A, thevertical axis shows the amount of shift in the threshold voltage(ΔV_(th)) of a transistor, and the horizontal axis shows sample name.

The results in FIG. 43A show that the amount of shift in the thresholdvoltage (ΔV_(th)) in the GBT stress test is small in Samples C1 and C2of one embodiment of the present invention. In particular, under photopositive stress, the amounts of shift in the threshold voltage (ΔV_(th))of Samples C1 and C2 is smaller than that of comparative Sample C3.

Thus, the transistors of Samples C1 and C2 of one embodiment of thepresent invention have small variations.

<Life Estimation of Device Based on Positive GBT Test>

Next, a positive GBT test was performed on Samples C1 to C3. Here, theconditions of the GBT stress test were as follows: the substratetemperature was 60° C.; the measurement environment was a darkenvironment; the gate voltage was +30 V; and stress time was varied.

The amount of shift in the threshold voltage (ΔV_(th)) was measured byvarying the stress time to 0.03 h, 0.14 h, 0.42 h, 0.56 h, and 1 h.

FIG. 43B shows ΔV_(th) under each conditions of stress time of SamplesC1 to C3, and approximate curves obtained from the values ΔV_(th). Notethat all the approximate curves in FIG. 43B are power approximatecurves. In FIG. 43B, a gray solid line is the power approximate curve ofSample C1, a black broken line is the power approximate curve of SampleC2, and a black solid line is the power approximate curve of Sample C3.In FIG. 43B, the vertical axis represents the logarithmic ΔV_(th), andthe horizontal axis represents the logarithmic stress time. In FIG. 43B,data and power approximate curves of Samples C1 and C2 substantiallyoverlap each other.

According to the results in FIG. 43B, the amount of shift in thethreshold voltage (ΔV_(th)) with respect to the stress time of SamplesC1 and C2 of one embodiment of the present invention is smaller thanthat of comparative Sample C3.

Thus, the transistors of Samples C1 and C2 of one embodiment of thepresent invention are highly reliable.

The structure described in this example can be used as appropriate incombination with any of the structures described in the otherembodiments or the other example.

Reference Example

Here, the oxygen concentration in an oxide insulating film and an oxidesemiconductor film in contact with the oxide insulating film wasevaluated by forming Samples D1 to D5 described below.

(Sample D1)

In Sample D1, an oxide semiconductor film was formed over a glasssubstrate. As the oxide semiconductor film, a 35-nm-thick IGZO film wasformed with a sputtering apparatus. The IGZO film was deposited underthe conditions where the substrate temperature was 170° C., a depositiongas (Ar/O₂=100/100 sccm (oxygen partial pressure=50%)) was introducedinto a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W wasapplied to a metal oxide sputtering target (In:Ga:Zn=1:1:1.2 in anatomic ratio).

Then, first heat treatment was performed. As the first heat treatment,heat treatment was performed at 450° C. for 1 hour in a nitrogenatmosphere and then heat treatment was performed at 450° C. for 1 hourin a mixed atmosphere of nitrogen and oxygen.

Next, an oxide insulating film was formed over the oxide semiconductorfilm. The conditions similar to those for forming the oxide insulatingfilms 214 a and 214 b in Example 2 were used for forming the oxideinsulating film. Note that in Reference Example, the oxide insulatingfilm is abbreviated as SiON or SiON film.

Then, second heat treatment was performed. The second heat treatment wasperformed at a temperature of 350° C. for one hour in a nitrogenatmosphere.

Through the above process, Sample D1 was formed.

(Sample D2)

In Sample D2, after the steps similar to those for forming Sample D1 areperformed, an ITSO film was formed over the oxide insulating film. Theconditions similar to those for forming the protective film 230 inExample 2 were used for forming the ITSO film.

Next, oxygen addition treatment was performed on the oxide insulatingfilms through the ITSO film. The oxygen addition treatment was performedwith an ashing apparatus under the conditions where the substratetemperature was 40° C., an oxygen gas (¹⁶O) at a flow rate of 150 sccmand an oxygen gas (¹⁸O) at a flow rate of 100 sccm were introduced intoa chamber, the pressure was 15 Pa, and an RF power of 4500 W wassupplied for 120 sec. between parallel-plate electrodes provided in theashing apparatus so that a bias would be applied to the substrate side.Since the SiON film included oxygen (¹⁶O) at a main component level, anoxygen gas (¹⁸O) was used to exactly measure the amount of oxygen addedby the oxygen addition treatment.

Through the above process, Sample D2 was formed.

(Sample D3)

In Sample D3, after the steps similar to those for forming Sample D2 areperformed, a step of removing the ITSO film was performed. Theconditions similar to those in the method for removing the protectivefilm 230 in Example 2 were used for the step of removing the ITSO film.

Through the above process, Sample D3 was formed.

(Sample D4)

In Sample D4, after the steps similar to those for forming Sample D3 areperformed, a nitride insulating film was formed over the oxideinsulating film. The conditions similar to those for forming the nitrideinsulating film 218 in Example 2 were used for forming the nitrideinsulating film.

Through the above process, Sample D4 was formed.

(Sample D5)

In Sample D5, after the steps similar to those for forming Sample D4 areperformed, third heat treatment was performed. The third heat treatmentwas performed at 250° C. in a nitrogen atmosphere for 1 hour.

Through the above process, Sample D5 was formed.

<Evaluation of Oxygen Concentration in SiON Film and IGZO Film>

Next, the oxygen concentration in the SiON film and the IGZO film inSamples D1 to D5 was evaluated. For the evaluation of oxygenconcentration, a SIMS analysis apparatus was used. FIG. 44 shows oxygenconcentration depth profiles of Samples D1 to D5. Note that the oxygenconcentration depth profiles in FIG. 44 are results of substrate sidedepth profile (SSDP)-SIMS.

As shown in FIG. 44, oxygen (¹⁸O) is not detected substantially in theSION film and the IGZO film of Sample D1 because the oxygen additiontreatment was not performed on Sample D1. Oxygen (¹⁸O) exists at only anatural abundance of 0.2%. Thus, the SiON film and the IGZO film ofSample D1 include little oxygen (¹⁸O).

In contrast, oxygen (¹⁸O) was detected from the SiON film of Sample D2subjected to the oxygen addition treatment. Furthermore, in Sample D3that was formed through steps in addition to the steps for Sample D2,oxygen (¹⁸O) is diffused to approximately 50 nm in the depth directionin the SiON film. However, oxygen (¹⁸O) is not diffused to the IGZO filmin Samples D2 and D3.

Moreover, in Samples D4 and D5 that was formed through steps in additionto the steps for Sample D3, since the nitride insulating film isprovided, the SiON film is heated at 350° C. when the nitride insulatingfilm is formed. Although oxygen (¹⁸O) is detected only in the SiON filmin Samples D2 and D3, this heat treatment makes oxygen (¹⁸O) diffuse tothe vicinity of the surface of the IGZO film or into the IGZO film inSamples D4 and D5.

As described above, by addition of oxygen to the oxide insulating film,the added oxygen is diffused toward the oxide semiconductor film.

This application is based on Japanese Patent Application serial no.2014-186092 filed with Japan Patent Office on Sep. 12, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A manufacturing method of a semiconductor devicecomprising the steps of: forming an oxide semiconductor film with asputtering apparatus; forming an oxide insulating film over and incontact with the oxide semiconductor film; forming a protective filmover and in contact with the oxide insulating film; and adding oxygen tothe oxide insulating film through the protective film, wherein theprotective film comprises at least one element selected from In, Zn, Ga,Sn, Ti, Al, W, Ta, and Mo, and wherein the oxide semiconductor film isformed at a condition in which an oxygen partial pressure in thesputtering apparatus is higher than 0% and lower than 50%.
 2. Themanufacturing method of a semiconductor device according to claim 1,further comprising a step of heating the oxide insulating film beforeforming the protective film.
 3. The manufacturing method of asemiconductor device according to claim 1, wherein an oxygen gas is usedfor a sputtering gas at the step of forming the oxide semiconductorfilm, and wherein the oxygen gas has a dew point of −40° C. or lower. 4.The manufacturing method of a semiconductor device according to claim 1,further comprising a step of heating the oxide semiconductor film beforeforming the oxide insulating film, wherein an oxygen vacancy is formedin the oxide semiconductor film in the step of heating the oxidesemiconductor film.
 5. The manufacturing method of a semiconductordevice according to claim 4, wherein the step of heating the oxidesemiconductor film is performed in an atmosphere in which an oxygenpartial pressure is lower than or equal to 1%.
 6. The manufacturingmethod of a semiconductor device according to claim 1, wherein the oxidesemiconductor film comprises In, Zn, and M, and wherein M is any one ofTi, Ga, Y, Zr, La, Ce, Nd, Sn, and Hf.
 7. The manufacturing method of asemiconductor device according to claim 1, wherein the oxidesemiconductor film comprises a crystal part.
 8. The manufacturing methodof a semiconductor device according to claim 1, wherein the protectivefilm further comprises Si.
 9. The manufacturing method of asemiconductor device according to claim 1, wherein the step of addingoxygen is performed with a plasma treatment apparatus.
 10. Amanufacturing method of a semiconductor device comprising the steps of:forming an oxide semiconductor film with a sputtering apparatus; formingan oxide insulating film over and in contact with the oxidesemiconductor film; forming a protective film over and in contact withthe oxide insulating film; adding oxygen to the oxide insulating filmthrough the protective film; removing the protective film; and formingan insulating film over and in contact with the oxide insulating filmafter removing the protective film, wherein the protective filmcomprises at least one element selected from In, Zn, Ga, Sn, Ti, Al, W,Ta, and Mo, and wherein the oxide semiconductor film is formed at acondition in which an oxygen partial pressure in the sputteringapparatus is higher than 0% and lower than 50%.
 11. The manufacturingmethod of a semiconductor device according to claim 10, furthercomprising a step of heating the oxide insulating film before formingthe protective film.
 12. The manufacturing method of a semiconductordevice according to claim 10, wherein the insulating film is a nitrideinsulating film.
 13. The manufacturing method of a semiconductor deviceaccording to claim 1, wherein an oxygen gas is used for a sputtering gasat the step of forming the oxide semiconductor film, and wherein theoxygen gas has a dew point of −40° C. or lower.
 14. The manufacturingmethod of a semiconductor device according to claim 10, furthercomprising a step of heating the oxide semiconductor film before formingthe oxide insulating film, wherein an oxygen vacancy is formed in theoxide semiconductor film in the step of heating the oxide semiconductorfilm.
 15. The manufacturing method of a semiconductor device accordingto claim 14, wherein the step of heating the oxide semiconductor film isperformed in an atmosphere in which an oxygen partial pressure is lowerthan or equal to 1%.
 16. The manufacturing method of a semiconductordevice according to claim 10, wherein the oxide semiconductor filmcomprises In, Zn, and M, and wherein M is any one of Ti, Ga, Y, Zr, La,Ce, Nd, Sn, and Hf.
 17. The manufacturing method of a semiconductordevice according to claim 10, wherein the oxide semiconductor filmcomprises a crystal part.
 18. The manufacturing method of asemiconductor device according to claim 10, wherein the protective filmfurther comprises Si.
 19. The manufacturing method of a semiconductordevice according to claim 10, wherein the step of adding oxygen isperformed with a plasma treatment apparatus.